St. Thomas, of course, lists five ways of presenting the one proof of the existence of God in the Summa Theologica, Q2, Article 3 (http://www.newadvent.org/summa/1002.htm#article3) He writes within the tradition of western philosophy, which recognizes the true, the good and the existent as only logically, not really, distinct. His five ways are logical perspectives of one argument, the singular conclusion of which is that there is a being whose nature and act of existing are identical. That Being, beyond our experience, must exist, in order to explain the existence of those beings which we do experience. It is telling that St. Thomas’ introductory counter to the objections to the existence of God is but one statement of reality, “It is said in the person of God, ‘I am Who am.’”
In western philosophy, the true, the good and the existent are called transcendentals, because they are only logically distinct. In human knowledge truth is the conformity of judgment to reality. In reality that which is true is that which exists. That which is good is that which is desirable. It is desirable to the extent that it exists. The beautiful is the good is the existent under the aspect of its attracting, rather than under the aspect of its being desirable. The beauty of reality attracts by the delight it offers to both the intellect and the will.
Western philosophy is based on two self-evident principles. A self-evident principle is one, which if denied, renders all knowledge and communication impossible. The first is: Things exist. The second can be expressed in many ways. Examples: Everything makes sense; Everything has an explanation; Reality is intelligible. The essential argument for the existence of God is that everything within our experience is limited in existence because it is explicable by its nature up to a point. Its nature explains its being in act, except for its very act of existence. There must be a being beyond our experience, which does not have this fatal flaw. Its nature must be to exist, thereby explaining its own existence and the existence of limited things.
St. Thomas’ five ways are:
1) Motion: There is motion from potentiality to actuality in the things within our experience.
2) The nature of the efficient cause: Nothing in our experience is the efficient cause of its own existence.
3) From possibility and necessity: The things we know are merely possible and as such are indifferent to existence.
4) There is gradation in the true and the good within our experience.
5) There is governance or purposefulness in things, even in inanimate things. Things move with purpose, i.e. toward the good. Purpose requires intelligence, which is lacking in inanimate things even though they are intelligible.
The first three ways refer directly to existence; the fourth refers to existence via the true and the good; the fifth refers to existence via the good and the true under the aspect of purpose.
That God exists, as a premise, cannot be proven. The existence of things is typically a matter of experience not a matter of a reasoned conclusion. However, God is nothing within our experience. Therefore, the existence of God cannot be known within the normal course of human experiential knowledge. Also, the nature of things is apprehended through experience. Since God is not within our experience his nature cannot be apprehended in the usual way. We cannot have a proper definition of a nature, which we do not know. Consequently, ‘that God exists’ cannot be an initiating premise to be proven because God, in an initiating premise, is undefined. For the same reason, ‘that God does not exist’ cannot be an initiating premise to be proven.
What can be proven is that there must be a being whose nature and act of existing are identical. This is a being whose nature, i.e. its definition, is To Exist. This being is the I AM. The starting point is that which is within our experience, that which we do perceive and know. The reasoned conclusion from what we do experience is that a being, outside of our experience, must exist whose nature and act of existing are identical. This being is I AM, whom we call God. However, it is only in this conclusion, and in understanding this conclusion, that we first appreciate the nature of a being who is properly called by the name, God. It follows that it is not possible to intellectually deny the existence of God, because it is only in understanding the conclusion, that I AM does indeed exist, that we initially have any proper definition for the word, God. Without coming to that conclusion, and thereby realizing its meaning, we have no adequate understanding of what ought to be called, God.
The five traditional ways of proof of the existence of God are variants on a single theme, which reaches the conclusion that a being must exist, whose nature and act of existence are identical. Emphasizing their variation on the theme of existence, we may re-list these five ways as requiring the explanation of:
1) The existence of beings moving from potency to act in the order of existence.
2) The existence of finite efficient causality in the order of existence.
3) The existence of unnecessarily existing beings.
4) The existence of imperfect beings in the order of existence.
5) The existence of purpose in finite beings.
The first way of argument concludes with the existence of an unmoved mover in the order of existence.
At first blush, the existence of an unmoved mover would appear to refer to motion from place to place, but it does not. Therefore, we must clarify to what level of motion the argument refers. It will be helpful to start with motion from place to place. Typically, we do not think of inanimate things as self-moving. (Even so, the most forceful motion form place to place with which we are aware is of inanimate things.) We recognize an animal as being alive because it can move itself from place to place. We may see a ladybug and think that it is dead, because it is not moving. Then we change our judgment when it flies away. It is self-moving at the level of local motion. Plants are not self-moving like animals at the level of local motion, but because they are alive they are self-moving at a higher level of motion than that of place to place. Plants are capable of assimilation. They take water and other inanimate materials and change them into their own plant substance. Plants are self-moving at a level higher than that of local motion. Yet, inanimate objects, which we judge not to be self-moving, are moved from place to place with great force. However, living things also are moved from place to place like inanimate things with great force. We may lose our footing and tumble down a hill like a stone that is dislodged. In both instances we judge the moved as not self-moving.
Humans have the capacity to move themselves at a level higher than that of other animals, namely, at the level of reason and will. This level of self-movement, although it can inform human motion from place to place, is a level of self-movement, which is far above the level of motion from place to place. (Note the gradation of self-movement is a form of gradation of perfection of being, as in way #4).
There is a greater level of motion than that directed by human intelligence. It is the motion of coming into being. Motion is the transition from potency to act, which is an immediate operation. Since things come into being and it would be a self-contradiction to claim that they moved themselves into being, there must be a being who exists in itself without any potency to become. That being is one whose nature and act of existing are identical. This being is fully act and is appropriately called, God. Where motion at all levels is movement from potency to act, in the being, I AM, there is no motion because he is fully act, fully being. Thereby He explains the motion of all other beings, quintessentially their motion into existence.
The second way of argument concludes with the existence of an uncaused cause in the order of existence.
Things exist and are inherently intelligible in their existence. On these two principles all human knowledge and communication depends. To deny either eliminates the possibility of human knowledge. To explain any aspect of reality is to identify its cause. Ultimately, the efficient cause of an existing event is the nature of that from which the event arises. (Examples: Assimilation is due to the nature of material things that are alive. Water evaporates. Water expands when it freezes.) The natures of things explain the events they produce. There is one event which cannot be explained by the nature of a thing in the order of efficient causality. That is the existence of the thing itself. The only possible efficient cause of the existence of those things that are within our experience, is a being outside of our experience who is the efficient cause of its own existence. The nature of such a being is To Exist. There can be no real distinction between its nature and its act of existence. The appropriate designation of this being is God.
The third way of argument concludes with the existence of a necessarily existing being.
The status of existence of anything is contingent upon that from which it derives existence. The current existence of everything and anything within our experience is contingent upon that from which it originates. All of the characteristics of everything and anything are adequately explained in their contingency upon the natures of things of which they are the characteristics. However, the very existence of every thing within our experience is not explicable as being contingent upon the nature of that thing. This is because the nature of nothing within our experience exists of necessity. The existence of everything within our experience is contingent. The contingency of its existence is only explicable by a being, who is not contingent. The only possible necessary being in the order of existence would be one whose nature is To Exist. Upon this necessary being in the order of existence, the existence of all other beings is contingent.
The fourth way of argument concludes with the existence of a perfect being in the order of existence.
We are aware of the gradation of beings within our experience of the inanimate and the animate and within the animate the gradation of plant and animal. Of course, we also recognize gradations within these grades. In the category of animal we recognize invertebrates and vertebrates, as well as many gradations within these. We recognize at least some of these gradations as indicative of degrees of perfection in being. We do not feel squeamish about killing a tomato plant or a housefly, but we are reluctant to kill a mammal. We forbid the killing of other human beings. This is not arbitrary. It is not anthropocentric, but the recognition that levels of life are levels of participation in being. It is the recognition of levels of perfection in being. However, everything within our experience is lacking in being in that it cannot bring itself into being or maintain itself in being. It is a lack of perfection common to all. This lack is evident in the indifference to being itself of the natures lacking the perfection of being. These beings can only share in being as not essentially their own, but as given to them. However, since everything makes sense, there must be a perfect being who gives being to those lacking the perfection of being. The perfection of being can only be had by the singular being whose nature it is To Exist.
The fifth way of argument concludes with the existence of a being, whose purpose is completely fulfilled in its own existence.
A being is alive if it is capable of acting for its own good or the good of another of its own kind. That level of being, which is life, is an end in itself. A living thing is capable of assimilation, making that which is not it into itself and also begetting others of its own kind. Animals are even more alive than plants because their capacity to act purposefully is enhanced by sensation and instinct. Humans are even more alive than other animals, being able to act purposefully under intellectual direction and will. The higher the participation in being, the greater is the participation in goal oriented action, in purposefulness. Yet, the greatest of these beings in intellectually goal oriented action, cannot sustain itself in purposefulness because it cannot sustain itself in existence. Even in a living thing’s apparently being an end unto itself, is not explicable as such because it cannot sustain itself in being. From its nature we must look for the explanation of its purpose and its existence in another. That other, which explains both the existence and purpose of the finite being, is the being who is The End in Himself and who is the cause of all existence and all purpose, because His nature is To Exist.
A sophisticated reader may disdain the proposition that there is purpose inherent even in inanimate things. Billy one day awoke in an opposite universe and enjoyed walking on the ceiling. However, it was already too late when he realized that going outside to play was a mistake.
The presentation by St. Thomas in Article 3 of the Summa Theologica could be misconstrued as proposing that God is the first in a series of movers, or the first in a series of efficient causes. The proper interpretation is that St. Thomas is rejecting a series altogether and identifying God as the unmoved mover and the first efficient cause in the order of existence. As such God is the immediate mover into being of all other beings and the immediate efficient cause of the being of each being. A series of movers or causes, each of which lacks the capacity to sustain itself in being but moving another into being or causing the being of another, makes no sense. Also, in his describing God in his earlier treatise, On Being and Essence, it is clear that only God, as pure act, as having no distinction between his essence and his act of existence, can give being to another. That giving of being is necessarily immediate, not intermediate through other beings. It may be in deference to Aristotle that St. Thomas expresses his argument for an unmoved mover and a first cause in terms of rejecting a series.
Finally, it should be noted that we often arrive at rational conclusions without adverting to the detailed steps of the rational process itself. Typically, if one were casually to identify what is meant by God, the answer would be the creator. We actually do have an adequate grasp for the meaning of the word, God, in recognizing the necessity of a creator. The creator is that being whose nature and act of existing are identical and therefore, who is the explanation, cause and end of all of the finite beings within our experience, which do not explain their own existence. God as creator is the conclusion of a rational process of thought, at which we usually arrive without adverting explicitly to the steps in that rational process. God as creator cannot be the initiating premise in a rationale demonstrating his existence, because creation is not fully understood until the nature of creator is reached as a conclusion based on the necessity of creation, i.e. the lack of an explanation in themselves for their own existence of the things within our experience.
Everything makes sense at the level of existence or nothing makes sense. A reflection of this truth is seen in the fact that one exception to the rules of logic would destroy logic in its entirety. For everything to make sense the existence of mutable beings, who do not explain their own existence, must make sense, must be explicable. Their existence makes sense, if it is granted to them by a being, who is the explanation of his own existence. This being is the One, whose nature and act of existing are identical.
In summary: Each thing within our experience has the same fatal flaw. Its nature does not explain its existence. There must be a being beyond our experience which does not have this fatal flaw because its nature is to exist. Consequently, this being
1) Is pure act. It does not move from potency to act. It does not come to be in any way.
2) Is the efficient cause of its own existence. It does not depend upon another for its existence. It is the immediate, efficient cause of the existence of everything else.
3) Exists of necessity. It does not depend upon another for its existence. They depend for their existence upon it.
4) Is the fullness of truth. Its nature fully explains its existence.
5) Is the fullness of the good. It is its own fulfillment, its own purpose, its own end, its own good, its own reason for being.
In the Judeo-Christian revelation of God, the ‘I am Who am’ of the Old Testament reverberates in the New Testament as ‘Before Abraham came to be, I am’. For those who believe, this corroborates that which is knowable by reason alone. Of course, reason alone cannot affirm the claim of Jesus of Nazareth to be I AM. However, reason alone affirms that I AM is and that there are five ways of reaching this conclusion.