The position of St. Anselm that God's existence is self-evident is sometimes presented as "the ontological proof": It would be impossible to have an idea of an infinite, perfect being if there were no such being really existing. The argument was taken up by Descartes, Leibnitz and Hegel. Its refutation by Thomas Aquinas, given above, is sufficient. Today no school of philosophical thought upholds the ontological argument.

Another argument, presented by many Ash`arite Muslim theologians, is based on the premise that the world must have had a beginning in time. It could not begin to exist by itself, but must have been produced by an eternal all-powerful being, which we call God. They attempt to prove the premise that the world had a beginning in time from the fact that a world that existed from eternity would imply an infinite series of night and day and of generation of men and animals. Such an infinite series, they say, is impossible. Therefore the world began in time.

This weakness of this argument is the premise that an infinite temporal succession is impossible. Such a series is not infinite in act, but only in potency. Here and now only a finite number of things exist. We cannot prove by reason that the world either had a beginning or did not have a beginning, but its creation in time can only be known from revelation. There is more discussion of this point in Section II, on creation.

The argument of Aristotle from motion is listed by Thomas Aquinas as the "first and most manifest" of his "five ways" [Summa theologiae, I, q.2, a.3]. It goes as follows: Everything that is moved is moved by another. That some things are in motion, for example, the sun, is evident from sense. Therefore, it is moved by something else that moves it. This mover is itself either moved or not moved. If it is not, we have reached our conclusion, namely, that we must posit some unmoved mover. This we call God. If it is moved, it is moved by another mover. We must, consequently, either proceed to infinity, or we must arrive at some unmoved mover. Now, it is impossible to proceed to infinity. Hence, we must posit some prime unmoved mover.

Aristotle argues for the proposition that everything that is moved is moved by another by pointing out that self-motion, applicable to animals, is possible only by one part moving another, and ultimately by the soul. He explains that violent motion, obviously, must come from an outside agent. Natural motion, however, such as the gravitational falling of bodies when an impediment is removed, proceeds from the substantial form of the body, which is an active principle of motion. It does not depend on any mover here and now, but only indirectly, in that the active form was given to it by whatever generated or gave it being. As a general principle, to be moved is to go from potency to act; yet nothing can be at the same time in act and in potency with respect to the same thing; therefore to be moved must mean to be moved by another.

Aristotle also argues that there can be no procession to infinity among movers and things moved. Such a series must be of bodies in contact with one another, so that they move are moved simultaneously as a single series. But if there is no first mover, all the other intermediary movers or instruments will not be activated, and there will be no motion.

The context of this argument becomes apparent from Aristotle's cosmological treatise, On the heavens, where he makes it clear that all life on earth depends on the sun. Its changing positions bring the wind, rain and heat that bring about all motion on the earth. Since he believed in an eternal universe, he held that the sun is incorruptible; it heats, but is not hot or on fir. Likewise he maintained that the sun's motion around the earth (according to the Ptolemaic theory) was not a natural motion, like gravity, but required constantly renewed energy to keep it going. Aristotle had no idea of impetus (or inertia), whereby an agent can communicate to a projectile a transient accidental form, resembling the permanent form of gravity, that keeps it in motion until this form is corrupted by resistance (Thomas thought of impetus, Commentary on the Physics, Book 7, lesson 3, but failed to apply it to the cosmos). Aristotle realized that no power could keep fuelling the sun, moon and the planets on their daily course around the earth for eternity unless it had infinite energy. Infinite energy cannot be contained in any body. Therefore the movers of these heavenly bodies must be spirits. These spirits carry out this task in service of the earth below out of love of the supreme principle of the universe, God himself.

Aristotle's universe consisted of a chain of movers depending here and now on a spiritual source. But once we introduce the notion of impetus, to say that the heavenly bodies are no different from man-launched satellites kept in motion by the two vectors of gravity and an impetus perpendicular to gravity, which need no refuelling but require only their initial propulsion, then there is no need to postulate spiritual forces to push the moon and other heavenly bodies. Once they were initially set in motion, they go on by themselves, just like natural motion, which Aristotle and Thomas Aquinas said requires no efficient cause here and now.

By the principle "Whatever is moved is moved by another" Aristotle and Thomas never meant that "Whatever is in motion is moved by another," but "Whatever is set in motion is set in motion by another." Modern physics shows that the heavenly bodies were set in motion in the distant past and need no additional energy to keep on going. Thus the argument from motion, as Aristotle constructed it, carries no weight. And once we try to construct a chain of movers going into the past we arrive at nothing certain.

Thomas' "second way" [S.T., I, q.2, a.3] is that a chain of efficient causes cannot go back infinitely, but must reach a first efficient cause, which is God. This argument is simply a rephrasing of the argument from motion in more general metaphysical terms, and it has the same weakness as that argument: In "sensible things" that we experience all the chains of efficient causality resolve in accidental dependence on causes in the past. A child depends on its parent for its becoming, but not for its present existence.

Many modern Thomists try to disengage Thomas' argument from motion from its cosmological context and reinterpret it in a broader metaphysical sense of "divine pre-motion". But this is to distort the actual thought of Thomas. It is true he had a place for "divine pre-motion", but his argument for a prime mover is fundamentally a physical theory based on his philosophy of nature.

Thomas' "third way" is taken from what is possible or necessary: We find some things that are possible to exist or not exist, since they are generated and corrupted. It is impossible for such things to exist forever, since what can cease to exist will one day not exist. But if everything has no necessity of existing, then at one time nothing must have existed. In such a case nothing could have come into existence, because everything must start from something. But, since things do exist, they must depend on something necessary. Some things, like incorruptible spirits, have a relative necessity for continuing to exist, but even these must depend on something absolutely necessary in itself, which we call God.

This proof assumes the important Thomistic principle of the real distinction between essence and existence in everything but God, a principle which goes back to Ibn-Sînâ and Boethius and even more remotely, but less clearly, to Aristotle. The essences of the things we know are in potency to the act of existence, which may or may not be present. All such things are contingent beings, dependent here, now and always on the direct action of One whose essence is identical with its existence to sustain it in being. Contingent beings depend on natural causes for their coming into being, but natural causes are restricted to the individual essences of their effects. For instance, a cat gives birth to this kitten, but in replicating itself the cat does not confer or sustain the existence of the kitten. Existence is a generic effect and must be referred to a universal cause, which is God.

In this sense, not only the substances of things but all natural causality and motion, as a sort of being, is immediately dependent on God, and we can say that he gives the power to act, preserves it, applies it to act and enters into the action itself. But he does not supplant nature on its specific level of causality; rather it is his instrument.

The root of the distinction between a contingent and absolutely necessary being is the fact that contingent beings have some measure of potentiality, whereas God, the absolutely necessary being, is completely in act. It follows too, as the argument from motion tried to show, that God is completely unchangeable and unmoveable, since any change or motion implies a transit from potentiality to actuality.

The argument from contingency supposes the fact and knowability of causality on both the sensible or physical level and on a metaphysical level, contrary to Hume's reduction of all causality to mere temporal order of succession.

Thomas' "fourth way" is taken from the gradation of being. Things have degrees of goodness, truth, nobility etc. But greater and lesser in any genus is always in reference to what is most in that genus. Therefore there should be something which is most perfectly good, true and noble, and consequently is most perfectly being, since being, truth and goodness are interchangeable. But what is greatest in any genus is the cause of what is less in that genus. So in the order of being in general, there must be a cause for the being, goodness and any perfection of all things. And that we call God.

Whereas the third way is based on the act of existence, this argument is based on the essential perfections scattered throughout the various species of being. All of these species have limited perfections, implying not only contingency of existence but also a dependency of sharing or participation in the perfect essence, which has all these perfections united together in the supreme degree (and consequently is not distinct from its existence). Thus God is the author or designer of each distinct species or nature, although individual natural causes multiply individuals within that species.

Thomas' "fifth way" is taken from design. St. John of Damascus proposed it [De fide orthodoxa, I, 3]; it was taken up by Ibn-ufayl (ayy ibn-Yaqân, pp. 176-177) and by Ibn-Rushd (Tahâfut at-Tahâfut, II, p. 647, 658; Manâhij al-adilla, p. 110; cfr. pp. 65-70, 77, 109-131). The natural things of the world, though lacking knowledge, act for a purpose. That they do so regularly and in the same way shows that they do not do so by chance but by intention. Since they do not have intelligence themselves, they must be directed by an outside Intelligence that orders all things to their proper goals, and that we call God.

This argument is based on two distinct areas of design; which one is more marvellous it is hard to say. The first is the internal order of any natural unit: Whether we examine a human body, an insect or a chemical, we find a complicated order of parts, elements and sub-atomic particles that baffles the mind, all working together to make the natural unit function well. The more one studies biology and chemistry, the more one is struck by the design found in nature.

The second area of design is the co-ordination of distinct natural units tform an eco-system that sustains life on this planet. It is the nature of a banana plant to produce bananas, but that they should be food for men is an extrinsic purpose. Emphasis on ecology in recent years has only highlighted the complex interdependency of all the different living and non-living components of this world. Thomas argues that contrary and discordant things cannot, always or for the most part, be parts of one order except under someone's government, which enables all and each to tend to a definite end. But that is what we find in this world. So there must be a God by whose providence the world is governed.

This argument has been contested on two fronts. The first objection is that it presupposes teleology as developed by Aristotle, which is the use of final cause in scientific explanation. Much modern thought, inspired by Darwin, denies final causality, purpose or function in nature, and explains everything by chance or the survival of the fittest, both in the evolution of natural species and in the order of the cosmos. Yet the order of the universe is too obvious to be denied. The role of science is to lend precision to sense data, not to deny it.

Another objection comes from the fact of evil in the world. On the level of the natural unit there is deformity, sickness and death. On the cosmic level there are earthquakes, plagues and other natural disasters that make human existence, at least, seem at the mercy of chance. Yet all these evils only go to prove that we expect health and order as a norm. The question of evil will be discussed in greater detail later, in connection with divine providence.