The "five ways"

FIRST PROOF OF GOD'S EXISTENCE, STARTING FROM MOTION OR CHANGE

The first and plainest way is the one that proceeds from the point of view of motion. It is certain and in accord with sensory perception that some things on earth are moving. Now, everything that is moving is moved by something.
Nothing, indeed, is changed, except it is changed to something which it is in potentiality. Moreover, anything moves in accordance with something in action: for to change something is nothing else than to bring something from potentiality into actuality, and nothing can be brought from potentiality to actuality except through something in action. Thus heat in action, as fire, makes fire-wood, which is hot in potentiality, to be hot actually, and through this process, changes it.
The same thing cannot at the same time be actually and potentially the same thing, but only in regard to different things. What is actually hot cannot be at the same time potentially hot, but it is possible for it to be at the same time potentially cold. It is impossible, then, that anything should be both mover and the thing moved, in regard to the same thing and in the same way, or that it should move itself. So everything can only be moved by something else.
If, then, that by which it is moved, is also moved, this must be moved by something still different, and this, again, by something else. But this process cannot go on to infinity because there would not be any first mover, nor, therefore, anything else in motion, as the succeeding things would not move except because of what is moved by the first mover, just as a stick cannot move anything except when it is moved by the hand. Therefore it is necessary to go back to some first mover, which is itself moved by nothing, and this all men know as God.

{{Scotus and Suarez both made their own objections against the conclusive force of the first proof of God's existence, as we find it with Saint Thomas.
It seems they understand the proof starting from motion after the formulation of the sources Thomas used, and not in the sense Thomas himself intended to give it.
Scotus and Suarez maintain we can only deduce from the local motion we perceive that there must exist, besides the movers we perceive, some higher active factor we can't experience, but not that we have to necessarily credit this higher mover with absolute immobility.

Saint Thomas derives his examples - the fire and the wood, the hand and the stick - from tangible reality in space and time.
But he doesn't stop there.
We wouldn't need to explicitly draw attention to this, if philosophers didn't repeatedly urge the same difficulty against the first thomistic proof of God's existence. They thought, because Aristoteles and his Jewish follower Maimonides (rabbi Moses) seemed to speak about local motion only, Saint Thomas would have done the same.
However, this opinion is definitely wrong.
Of course, every example can only partially shed light upon the general law. Moreover, the given argumentation clearly shows Thomas doesn't speak of local motion only, but of each motion, or better, motion in the metaphysical sense. In all circumstances, it is a transition from potentiality to actuality. That is to say, all that is moving or changing - it doesn't matter whether it is a change of locality or species or substance - takes a new shape, which it didn't actually have before.
If we understand moving in the intransitive sense, it means becoming different. Not being so and so, but becoming so. Motion is intermediate between being so and becoming something else. Therefore, we can justly define it as potentia in actu (potentiality on its way to actuality) or actus in potentia (completion of the potential state). We can say about a thing that changes that it is on its way to some completion according to a certain potentiality. Motion is neither pure potentiality, nor pure actuality or completion, but somewhere in between.
Moreover, we have the principle that anything that is moving, doesn't move its own self, but is moved by something else. We can answer for this datum by reference to the principle of causality. Indeed, the things that are moving or, what amounts to the same, changing, aren't identical with their actual form, but they have only the potentiality to become so. Because the completed form is not their essence, some external cause must bring about the connection between the things and their completion. But when something that is in some respect only potential, can't give the completion or actual sate to its own self, then an influencing cause has to bring the completion to it, so this external cause must actually have the completion. The latter may have received this completion from yet another cause in its turn, but we can't proceed in infinity ...
There must be a limit somewhere, although this limit may not exist in space or time.
Eventually, we'll have to acknowledge the actual existence and highest completion of a first mover, even though we do accept the possibility of an infinite sequence of moved movers. The first mover exceeds any sequence. It is unmoved and immobile itself, and the cause of all motion of things in every sequence.}}

SECOND PROOF OF GOD'S EXISTENCE, STARTING FROM THE EFFICIENT CAUSE

The second proof is from the nature of the efficient cause. We find in our experience that there is a chain of causes. However, we don't experience, nor is it possible, anything would be the efficient cause of itself. In fact, such a thing would have to exist before itself, which is impossible. In the case of efficient causes, the chain can't go back indefinitely, because in all chains of efficient causes, the first is the cause of the middle, and the middle of the last, whether there be one or many middle causes. In any case, if the cause is removed, the effect is removed. Hence if there is not a first cause, there will not be a last, nor a middle. But if the chain were to go back infinitely, there would be no first cause, and thus no ultimate effect, nor middle causes, which is evidently false. Hence we must presuppose some first efficient cause, which all call God.

{{The first proof of God's existence started from the changed thing, or, to be sure, change in general as a transition from potentiality to actuality which we see in all changed things. The second argument for God's existence is based on the reality of the efficient causes we experience and perceive. They only can make their causality felt because former causes enable them to do so.
Many people represent this demonstration as the most understandable one. And justly so!
Especially since we know the distinct causes are dependent on each other. They are causae per se ordinatae - causes that form a serried sequence by their very causality and efficiency.
Applying the law of causality and the principle that we can't proceed till infinity, we conclude there must be a first cause that is not caused itself and yields an explanation for all causality outside itself.
Indeed, in the efficient cause we consider causality itself rather than the thing that's working as a cause. We have to admit such a cause isn't causality itself, but is sharing its causality with former causes. Any thing is, as a cause, dependent on a former and higher cause, which we have to explain in the same way. The serried sequence of caused causes can be very large indeed, but is caused itself. So ...
Let's not forget we can't fully experience the serried sequence of causes influencing each other's causality.
De Vries SJ says: "Der zweite Weg wird gewöhnlich als Beweis aus den wesenhaft geordneten Ursachenreihen (causae per se ordinatae) aufgefaszt: Jede Ursache, die nur in Kraft einer höheren Ursache wirkt, setzt notwendig eine höhere Ursache voraus. Nun steht es aber noch nicht ohne weiteres aus der Erfahrung fest, dasz jede innerweltliche Ursache causa per se ordinata ist, das heiszt, nur in Kraft einer höheren Ursache wirkt; ... darum führt dieser Gedankengang soweit er hier ausgeführt ist, noch nicht zu einer unbedingt überweltlichen Ursache."
However, whether we experience it or not, the serried squence of causae per se ordinatae can't but exist. Because it is certain that in the thing of nature causality is only existing by participation, so it is presupposing in the beginning of a long or short sequence of causes a highest cause which is cause by essence.
We have to distinguish the conclusion from the demonstration by means of efficient causes from the conclusion of the first proof. The conclusion is, there must be a first uncaused cause that explains causality in all other things.}}

THIRD PROOF OF GOD'S EXISTENCE, STARTING FROM THINGS CONTINGENT

The third proof is taken from the natures of species temporary and species eternal. We suppose that certain species of things exist only during a certain era. Since there is only a finite number of temporary species, if all species are temporary, there must have been a point of time when nothing existed. If this is true, even now there would be nothing, because what does not exist, does not take its beginning except through something that does exist. If then nothing existed, it would be impossible for anything to begin, and there would now be nothing existing, which is evidently false. Hence not all species are temporary, but there must be at least one eternal thing. Now every eternal thing either has a cause of its eternal existence, or has not. If it has a cause, it is contingent, like anything temporary. In the case of contingent things, the chain of causes cannot go back infinitely, as we have seen before. Hence there must be something eternal through its own nature, not having a cause elsewhere but being itself the cause of the existence of all other things, which all call God.

{{This demonstration has in the given formulation a remarkable difficulty.
The formulation uses the concept of species, which is rather vague. If we mean a kind of things which we can describe with at most a thousand words, it is clear there is only a finite number of temporary species, and anything we observe belongs to some species of things.
How did Saint Thomas arrive at this proof? Did he copy Maimonides' contingency proof without much criticism? It is possible, but he may also have found the reference to temporary things useful, because everybody understands that a thing which is not contingent must exist in eternity and that contingent things can't exist if there is nothing not-contingent. So Saint Thomas starts from the metaphysical constitution of things contingent.
This is the third time Saint Thomas starts from experience. This time he bases his proof on the contingentia, the things contingent and precarious. They form a firm base for the metaphysical proof which leads us to the ens necessarium, the thing that can't but be.
Indeed, by sensory perception we all know the things in our visible and tangible world come into being and perish. We can deduce without difficulty they are only precarious things and their existence is contingent.
But whatever can as well be as be not, is not essence itself; it only participates in being.
So, to explain existence in a contingent thing, there must be somewhere an external cause . The immediate cause could have a contingent existence itself, and, thus, presuppose in its turn another external cause.
However, the sequence of contingent causes must be finite, that is to say, outside and above the sequence there must be a first cause which is essence itself.
We may and must accept a world which has a contingent existence both in its parts and as a whole. But it would be absurd to only accept such a world. Because the cosmos we live in can't exist if there isn't above the cosmos a first cause that is essence itself.
Kant didn't understand very much of the contingency proof. He thinks it's not essentially different from Anselmus' proof of God's existence. He accounts for this in the following way:
"Um seinen Grund recht sicher zu legen, fuszet sich dieser Beweis auf der Erfahrung und gibt sich dadurch das Ansehen, als sei er vom ontologischen Beweise unterschieden ... Dieser Erfahrung aber bedient sich der kosmologische Beweis nur, um einen einzigen Schritt zu tun, nämlich zum Dasein eines notwendigen Wesens überhaupt. Was dieses für Eigenschaften habe, kann der empirische Beweisgrund nicht lehren, sondern da nimmt die Vernunft gänzlich Abschied und forscht hinter lauter Begriffen: was nämlich ein absolutnotwendiges Wesen überhaupt für Eigenschaften haben müsse."
Apparently, Kant only knows the 'cosmological' proof of God's existence in the strange variants of Protestant theologians like Wolf and Baumgarten.
Anyway, Kant has a wrong judgement of the thomistic proof of God's existence, starting from the contingent things. In this proof, as opposed to Anselmus' ontological proof, the intellect stays in touch with experience.}}

FOURTH PROOF OF GOD'S EXISTENCE, STARTING FROM THE DEGREES IN THINGS

The fourth proof arises from the degrees we find in things. For we find in them a greater or smaller degree of goodness, truth, nobility, and the like. We call the degree of such a property greater as the thing comes closer to something that is greatest possible with respect to that property; just as, for example, we call something hotter if it comes closer to greatest possible heat. Therefore, there must exist something that is goodness, truth, nobility, itself. Because metaphysics says, whatever has the greatest share in a property, is most like the essence of the property. Now if something is greatest in its kind, it must be the cause of the property in all other things of the kind; thus, fire, which is greatest possible heat, is the cause of all heat. Therefore there must exist something that causes in all things its existence, goodness and every perfection whatsoever, and this we call God.

{{The fourth proof of God's existence, as given above, makes us think of Plato's theory of ideas. Saint Thomas certainly took advantage of it. But, besides, he used the data he found with Aristotle and the Arab philosopher.
Again, the reasoning starts with experience. It's the experience of all things in this world, which have in a greater or smaller degree the transcendental properties of being. They all have their own place in the wide scale of being, as inanimate matter, plant, animal or man.
We may first find it strange when somebody says that being can come in degrees, because, apparently, the existential possibilities of any thing don't admit other variations than to be or not to be, yet an imperfect participation in perfect being is not a contradiction.
It's absolutely true that being as such is perfect. The properties we ascribe to being, are also perfect. For being is actuality, and hence perfection. Indivisibility, truth, nobility, goodness and beauty themselves are included in highest actuality, completion and perfection.
We would be foolish if we were to maintain all things possess being and its properties in the same degree. For being is in distinct things restricted in distinct ways. This restriction of being doesn't find its base in being or its properties, but in the circumstance that the metaphysical composition of potentiality and actuality in a datum of experience gives being an analogous character.
For instance, we can't say about a good man he is good in a less degree as a pure spirit on the strength of the character of goodness itself. And it's not indivisibility as such that makes it appears more in man than in animal. If pure spirit and man were only good, or if man and animal were only indivisible, there wouldn't be a gradual distinction in goodness between man and pure spirit, nor a distinction in indivisibility between man and animal.
The truth is: we can't say about anything that's composed of potentiality and actuality that it simply is. It has a share of being, by participation. And because a gradual difference in being is only possible by this character of participation, we can't accept distinctions in the degrees of indivisibility, goodness, and further properties of being as such , but for this reason only.
The fourth proof of God's existence starts from these distinctions we perceive in the measures of being, to conclude there must exist a highest being, which is the very essence of being, and is transcendent above all what has a share of being by participation, and explains the properties of being in all other things.
Because there exists a distinction in the degrees of being in distinct things, there must exist a being we find partially in these things. We can only explain the presence of partial being by the existence of a being that is the essence of being.
There is no methodical distinction between the fourth way and the first three ways.
Again, we start with experience. Reason forms the righteous judgement that the things we perceive with our senses aren't being without further preface, but are participating in being in distinct degrees. This character of sharing is necessarily presupposing an external cause. But we can't proceed in infinity. So ..
Kant didn't sufficiently know the demonstration from the degrees of being. And later kantians didn't pay much attention to it.
But Edouard Le Roy, who can't agree with any thomistic proof of God's existence, has been storming at what he called "the rationalistic idea of a God-ideal". He thinks scholasticism has tried in vain to prove the existence outside the intellect of an "idéal de sagesse ou de beauté". Furthermore, he says: "s'il se borne aux horizons terrestres, il succombe avec l'individu".
Le Roy, too, commits the error he simply identifies this intellectualistic argument for God's existence with Anselmus' proof of God's existence.}}

FIFTH PROOF OF GOD'S EXISTENCE, STARTING FROM THE ORDERING OF THINGS IN ACCORDANCE WITH A PLAN

The fifth proof arises from the ordering of things. For we see that some things which lack reason, such as natural bodies, are operated in accordance with a plan. From this it appears they are operated always or mostly in such a way that they attain what is the best. Whence it is clear that they do not arrive at the result by chance but because of a purpose. The things, moreover, that have no intelligence don't tend toward a result unless directed by something knowing and intelligent; just as an arrow is sent by an archer. Therefore there must exist something intelligent by which all natural things are arranged in accordance with a plan, and this we call God.

{{To really understand the fifth proof of God's existence, we have to realise what appears to be simple experience for Saint Thomas.
Indeed, Thomas says things in nature are either always or mostly operated in accordance with a plan, to attain what is best; which we can't explain by chance, but only by purpose.
Here again, the man from Aquino demonstrates he is a true disciple of the man from Stagira, because both deduce finality in nature from the perceivable fact that nature is always or usually operating in the same way.
Whatever a certain thing of nature does regularly, we can't explain by reference to chance. Because chance is exception. But whatever a thing in nature only does in an exceptional case, presupposes the influence of an external cause that occurs by chance.
Aristotle elucidates with an example what he understands by nature and chance.
When, in winter, it's often raining, that's no chance; but when it rains during the dog-days, that's chance. On the other hand, it's no chance when it's hot weather during the dog-days; but when it's warm in winter, that's chance. We may conclude nature is operating regularly, but it allows exceptions. As opposed to this, chance is a spoil-sport; as an accidental cause, it operates irregularly.
We know the nature of any thing by the regularity with which it is operating in a certain direction. For example, a swallow builds its nest always in the same manner, and plants of a certain kind produce regularly the same blossoms and the same fruits.
Now a consequence that's returning always or mostly in the same constant way, must be an intended consequence; that is to say: the regular consequence is the very purpose. For the regularity in the consequence presupposes a regulation in the cause, and regulation presupposes intelligence.
However, we can't find any explanation for the dynamic finality reasonless nature shows in its regular activity. It does possess this bent to its destination, but we can't say more. The question of the efficient cause that effectuated this finality in nature, remains completely open.
Neither can chance explain the regularity in the consequence, or be the cause of the finality in nature. For cause is opposite to regularity; in fact, it's infringement of the regularity, which allows an exception.
So we can only explain the regularly returning consequences and the finality in the distinct things of nature that effectuate these consequences by referring to an intellectual being that exists outside nature but operates within nature.
Where is this intellectual cause that is regulating things in nature and cosmic nature as a whole to make them strive after a purpose?
Is it a man? A spirit?
When experience isn't clear anymore, we could think several hypotheses are possible.
If we want to seek the immediate cause of the planned ordering of nature in an intellect that is dependent on a higher cause itself, we can first make the objection that the idea of the demiourgos, the imperfect architect of the world, could only originate from the imagination of thinkers who found the immeasurable distance between God and world too great to bridge at once by one single deed.
But nothing measurable can bridge this immeasurable distance!
Furthermore, did somebody ever sufficiently elucidate the big enigma concerning the existence of the world or the little one concerning the existence of a thing in nature by reference to the dependent architect of the world?
In the sequence of dependent planners, we can't proceed till infinity, either.
In the final end, we can only sufficiently explain the objective of the distinct planners of nature and the objective of nature as a whole by a perfect intellect, outside and above all dependent things, which doesn't need any objective nor any higher planner, but is planner and objective itself of all things operating in reasonless nature.
"Nature is a divine art", says Saint Thomas, not without a reason, in his comments on Aristotle's Physics.

The philosophers couldn't unanymously agree with the inner ordering of things in reasonless nature in accordance with a plan.
For instance, Leibniz had to defend the inner finality of things of nature against the mathematical explanations of nature given by Descartes and Spinoza, who had so much exclusive interest in the principle of formality that they didn't see the principle of finality any more. And Emile Boutroux had to scientifically undo the main thesis of some pragmatists and all evolutionists by arguing a bird doesn't only fly because it has wings, but also has wings to fly with.

Again, the most radical condemnation of the "physicotheological" proof of God's existence comes from Emmanuel Kant. This time, he begins with a deep bow:
"Dieser Beweis könnte höchstens einen Weltbaumeister der durch die Tauchlichkeit des Stoffs, den er bearbeitet, immer sehr eingeschränkt wäre, aber nicht einen Weltschöpfer, dessen Idee alles unterworfen ist, dartun, welches zu der groszen Absicht, die man vor Augen hat, nämlich ein allgenugsames Urwesen zu beweisen, bei weitem nicht hinreichend ist."
We hear the same sound again from Brunschvicg:
"Nous nous sommes soumis à l'obligation de proportionner la divinité à ce que le monde en rélève, avec le risque de dégrader Dieu et de rabaisser en nous son idée. Le Deus artifex sera aussi loin que possible du Deus sapiens qu'on aurait voulu découvrir et vénérer."
If scholasticism would stop half-way, and be content with a higher being which had only enough wisdom to bring about the still restricted finality of things of nature, the kantians would be right. But scholasticism doesn't stop half-way!}}

The overall conclusion of the "five ways"

Saint Thomas' proofs of God's existence all start from a common base: that all things in the world are present by participation. The proofs lead via the principle of finality to the same conclusion: God is the One who exists without further preface.
Nobody can undo this systematical unity of base, argumentation and conclusion by reference to the eclectical character of Saint Thomas' work. He incorporated each one of the five ways in his system and gave them a direction they didn't have in their original forms.
We readily admit the man from Aquino gratefully made use of existing traditions when composing his five arguments; he borrowed much from former philosophers. He learnt from his teacher Albertus Magnus how Aristotle and the Arab philosophers found in change, causality and finality a firm base for their reasoning that concludes there must exist a first, unchangeable and independent intelligent Cause. Maimonides showed him the contingent character of things in nature. Anselmus showed him the gradual distinctions in being we can easily recognize in the realities we experience. These distinctions are offering reason a certain guarantee for the rightness of its conclusions about the necessity and eminence of divine Essence.
However, it would be erroneous to think Saint Thomas blindly copied the proofs of God's existence from other writers. The proofs do indeed show they are dependent in many respects, but in their actual purport they bring something essentially new. In fact, Saint Thomas only borrowed the principles from his famous predecessors, not the elaborated theses.
For example, there is a considerable difference between Saint Thomas and Aristotle as to the concept of God. Against the unpersonal and lifeless supreme being of the man fom Stagyro, the man from Aquino sets the personal, living and active God "Whom all men know". He broadened Maimonides' rather narrow idea about material spheres to the concept of the whole cosmos as composed of potentiality and actuality. He couldn't begin anything constructive with the proofs of God's existence from Plato and Saint Augustine before he enriched them with the principle of efficient causality. Like Saint Augustine, Saint Anselm, Hugo of Saint Victor, Saint Bonaventura, and other representatives of the Augustinian way of thinking, Saint Thomas readily admits there must exist something, at the end of the gradual sequence of things good and true that approach the supreme being ever nearer, that is most perfect in every respect. But he isn't content with that. To be sure about God's existence and transcendence it's not sufficient He is the highest conceivable in a certain sequence. We have to know Him also as the cause without a cause of the things within the sequence that exist on a absolutely lower level, the level of nature we can experience, and are in some degree similar to Him by the properties they share with His transcendental essence.
Furthermore, the thomistic proofs of God's existence are relying on the metaphysics of the things outside us. They are resting on external experience, not on the inner experience of a philosopher. Thus, the thomistic standpoint is in the first instance essentially different from, for example, Saint Augustine's standpoint. Indeed, the great Church father thought he could find sufficient guarantee for the existence of God as highest Truth in the knowledge of illuminated truth and in the datum of experience as an example of this truth.
Saint Thomas doesn't show much interest for the logical certainty of the knowing intellect. He thinks it's too difficult to go from personal perception to infinity with one single step. He also knew the deontic proof of God's existence, starting from conscience, and the eudaimonic one, starting from the longing for happiness, but passed over them, because besides supernatural Revelation only the metaphysics of being can give us certainty about the existence of a highest and transcendental Being.
The endpoint where all ways come together, is Being itself. It's being in so far as it's existing in itself and on the strength of itself, not in so far as we find it in several things while sharing its properties with something greater by participation. Therefore, the most suitable name for God's Essence is: Qui est - Who is.
The distinct properties of this being, like being the unmoved force of moving, being the uncaused cause, being necessary, transcendental, good, true, beautiful, being the highest intellect and the final destination, all these properties are in our view distinct from each other and from divine Essence, but in reality they coincide in God, because, by each separate conclusion, they belong to God not by participation but essentially.

Other proofs for God's existence?

The five ways prove God's existence. They give us the high certainty we can get through the causes and the consequences. The visible world we live in presents itself as God's creation. It points in five ways to a first Cause that on closer investigation turns out to be our God of Christianity.
We can pose the question whether these five classical arguments exhaust the supply of all possible a posteriori proofs. As soon as we discover next the five given modalities some other universal modality of being that demands a trancendental cause, we can answer the question in the negative.
However, if we consider the overall history of philosophical and theological thinking, we see that human intellect in course of time didn't often seriously try to find a new proof of God's existence based on the principle of causality, but instead to find a completely different scholarly proof, independent of efficient causes.
Among the many endeavours to prove God's existence without climbing from the caused datum of experience to the first efficient cause, two stand out above all other ones in history. These are the proof of Anselmus, which many people call quite unjustly the ontological proof, and the proof that starts from man's natural pursuit of happiness, which is becoming very popular in our days.
Both proofs can refer to the conviction of clever thinkers. Furthermore, they have something enchanting. They are at first sight less intellectualistic than Saint Thomas' five ways, and are speaking to us more directly.

Anselm's (false) proof

This proof received great fame in history. Anselm was the inventor of it. He formulated it this way:
"That which we can't think anything higher above, certainly can't exist in mind only. Because, suppose it exists only in mind. Then we can think it exists in reality, too, which is higher. So, if that which we can't think anything higher above exists only in mind, then that which we can't think anything higher above is at the same time something which we can think something higher above. However, such a thing isn't possible. Hence, without doubt, that which we can't think anything higher above must exist both in reality and in mind."
The munk Gaunilo, a contemporary of Anselm, already opposed this argumentation in a publication he called the Book for the fool. Gaunilo argues Anselm's demonstration won't convince the fool who doesn't know God exists both in reality and in mind. He illustrates this with the wellknown example of the utopian island which is the most beautiful place we can think about. But we can only believe it really exists until somebody proves the opposite. According to Gaunilo, if somebody maintains he's crediting something with existence in his thoughts, which doesn't exist in reality, we can't say anything against it.
Anselm's proof received the most complete refutation from Saint Thomas Aquinas, who expressly dissents from every attempt to conclude God really exists, starting from the concept "God" only. Thomas supposes Anselm didn't want to prove God actually exists, but only that we have to include in his concept of God the very existence of God. In fact, this concept is "that which we can't think anything above".
It's true: the existence of God belongs to His essence. Therefore, the definition of God's name necessarily includes the existence. But we only know something really existing corresponds to the definition of the name, when we can conclude that in some other way. It's no inner contradiction when we accept that the concept "God" implies his existence, without knowing that He actually exists.

{{Saint Thomas' refutation of Anselm's proof for God's existence didn't stop all later philosophers from standing up for the rights of the a-priori argument.
For instance, Scotus tried (in vain) to save Anselm's reasoning by a sharp distinction, meant to make us understand Anselm didn't intend to say we know the idea "God exists" from the concept "that which we can't think anything higher above" alone.
According to Scotus, our knowledge about the existence of God is based on two distinct syllogisms that yield a joint indisputable justification of the conclusion that "that which we can't think anything higher above" must exist in reality.
The first of these two syllogisms is: "Whatever is existing is more than what doesn't exist. The highest has nothing above it. Hence the highest must exist." The second syllogism is: "Whatever doesn't not exist, does exist. The highest doesn't not exist. Hence it does exist." (But again, we can't speak about "the highest" before we know it exists.)
However, Scotus was sensible enough to understand that this formalistic way of thinking alone doesn't make us conclude the infinite Essence must exist. Therefore, he coloured Anselm's original proof of God's existence as follows:
"If God as the infinite Essence is possible, He does exist. There is no contradiction whatsoever in the concept of the infinite Essence. Hence God must exist." (But why should something exist only because it is possible.)

Descartes has an explanation of his own for Anselm's proof of God's existence. He tries to make it plausible by reference to God's necessity: "To say something is included in the nature or the concept of a thing, is equal to saying it's true. Now the idea 'God' necessarily implies his existence. Therefore it's true when we say about God that in the existing idea of Him, His existence is necessarily included." (Thus he relies on the existence of the idea, which is prevailing all over the world indeed.)

Leibniz couldn't unconditionally approve of this argumentation, because in fact it starts with presupposing God is possible.
"I diligently studied Descartes' reasoning, and I conclude we can at least deduce with certainty from the given argument that the existence of God is sure as soon as we prove He is possible."
To give it a more stringent conclusive force, Leibniz therefore coloured the argument and found a complementary reasoning which, in fact, is the same as the third scholastic proof of God's existence:
"God must be possible because He is necessary, and He is necessary because, without Him, there would not be a sufficient base for the existence of the things contingent."

Notwithstanding the many colours of the a priori proof of God's existence, our fundamental objection against it remains unabated.
Anselm thought the existence is included in the concept of the highest, whilst Scotus and Descartes deduce the existence of God from the definition of His name and the concept of Himself. This distinction is not an essential difference. Because, whether we think to establish the existence directly or indirectly, by simple analysis or by syllogistic reasoning, in the end it turns out we start from the definition of God's name. As long as the existence or possibility of God doesn't appear from an existing reality that we experience, which shows a dependence on one of the qualities of God, even a coloured variant of Anselm's proof isn't sufficient at all.
Leibniz understood this very well. His redaction of Anselm's proof clearly shows that the a posteriori argument starting from the contingency of the things in the world has to give complementary strength to Anselm's reasoning.
God exists, says Leibniz, because He is possible. And He is possible, because He is the Ens necessarium ....
Leibniz could have confined himself to this last remark. Because we needn't prove the existence of anything that exists of necessity by reference to its possibility.}}

The proof of God's existence that starts from our natural longing for happiness

Some people called this proof the sixth way, because it doesn't start from the concept of God, like Anselm's, but from a perceived phenomenon in our experience, and because it seeks a sufficient base for a certain modality of being in a cause above the cosmos.
However, there is an important distinction between Saint Thomas' classical arguments and the demonstration from our natural longing for happiness, which is from a much later time, at least in its sharpest formulation. The distinction is much less in the difference of time than in the fact that, as opposed to the five ways, this new argument intends to conclude there must be a final cause above the cosmos rather than there must be a transcendental efficient cause.
In a syllogistic form, the sixth proof of God's existence looks like this:
"We perceive in ourselves a natural longing for happiness. A natural longing can't be idle. Therefore, God must exist as our final destination."

{{There is an intended variant of the sixth way, the so-called metalogical proof of God's existence, which we can't accept as a fit substitute but under strict reserve only.
It does start from the natural longing in man for unrestricted final perfection of our personality, but can't logically convince us of the real existence of it's final term: the infinite principle of perfection.
Gratry was the great defender of this variant. He thought he could refer to the authority of Plato, Aristotle, Saint Thomas, Descartes, Malebranche, and others. But this didn't imply he could find substantial approval in the circle of his contemporary philosophers.
His reasoning was as follows:
"Tout est fini sur la terre, mais j'éprouve une tendance irrésistible vers l'Infini et le Parfait. Donc il existe l'Etre infini et parfait."
There is something missing in the logical construction of this demonstration.
Indeed, from this single premiss we can hardly deduce the conclusion God exists. Gratry forgot something. He should have proposed a second premiss: that a tendance irrésistible can't be idle.}}

If we leave out of consideration the distinct arguments for God's existence that are meant to be original on the strength of their reference to finality, but in fact coincide with the third or fifth way, the via sexta appears in only two more shapes with conclusive force.
In the first place, we have the demonstration that understands natural longing in the sense it coincides with the whole striving efficacy of our will or with the act of longing that our will must of natural necessity perform. In both of these cases we are supposed to perceive in our mind the good we naturally strive after as a happiness which is not within the independent reach of our abilities. Since this natural longing can't be idle, and only God can give this perfect happiness, it's clear God must exist.
In the second shape, the via sexta doesn't understand natural longing as a single act or a complex of acts, but as the principle that is the foundation of all striving efficacy. For example, Zimmerman SJ understands natural longing as an ontical ordering of the nature of reasonable creatures, whilst Gredt OSB emphasizes the ontical inclination of the nature of our will as an ability. In both of these cases, the object of our natural striving isn't a good we perceive in our mind, but the highest perfection of ourselves we "blindly" are striving after.

The proof of God's existence starting from man's natural longing for happiness certainly yields as strong a degree of probability as, for instance, the ethnological proof, that starts from the fact that all peoples acknowledge the existence of a supreme God. Yet we think it's too weak, at least in its present interpretation, to convince us of its metaphysical conclusive force.
Against the sixth way in its first shape, we are aware of the difficulty it concludes too readily that the object of natural longing must imply the existence of God. Because the efficacy of our will is entirely directing at the bonum in communi, the good wherever we may find it. We can only know that divine Good is included when the existence of God is no longer a problem for us. The object of the necessary act of our natural will is a good that is necessarily related in our mind to happiness including at least existence, life and knowing, because we have to call human happiness a fiction without these things. But our intellect needs not relate happiness to a predetermined object like God.
As to the argument in its second shape: is it immediately clear that the infinite good we're blindly striving after, is identical to the infinite essence of God? Somebody who doesn't know God exists can hardly think the infinite is anything different from the whole of all things existing, including the cosmos.
However, the aristotelian principle of finality has a similar difficulty. We are tempted to deduce the conclusive force of this principle from God's omnipotence and wisdom, which doesn't create things lacking a purpose or a sense. But whoever is referring to the existence of God to justify the value of the principle of finality, can't prove the existence of God with the help of this principle.