Aquinas thought that reflection on familiar features of the physical world affords evidence of God's existence. So he attempted five proofs to demonstrate the existence of God. Recall that Anselm had begun his ontological proof with the idea of a “greatest conceivable being,” from which he inferred the existence of that being. By contrast, Aquinas said that all knowledge must begin with the experience of sense objects.

Before addressing Aquinas's five arguments for the existence of God, one thing is worth noting. The question of God's existence is a metaphysical and epistemological concern, since it raises the question of what kind of reality God is and if you can know — and how you can know — whether that reality exists.

Like Aristotle, Aquinas was an empiricist, claiming that knowledge comes from experience. By contrast, Anselm was a rationalist. For Aquinas, sense experience tells you that this universe is a system of causes and effects and lawlike behavior. This world system, this cosmos, requires a transcendent cause. Aquinas's five arguments fall into several categories. It is instructive to examine arguments one, two, and five.

Causal Arguments

In the first way, Aquinas writes that “it is certain, and it is clear from sense-experience, that some things in this world are moved.” Aquinas, like Aristotle, understood the term motion to be reduction from a state of potentiality to one of act. If something is in motion, it must have been moved by another, and that by another, and so on. But this series of motions could continue infinitely, according to Aquinas, “for then there would be no first mover.” And then there would be no subsequent movers, since subsequent movers move only inasmuch as they are moved by the first mover; “the staff moves only because it is moved by the hand,” Aquinas explains. “So there must be a first mover, moved by no other; and this everyone understands to be God.”

From the first argument one can observe a feature that appears in all five. Aquinas ends each of the arguments with the conclusion that God is the cause of some reality, since without God the reality would not be explainable.

“The second way is from the nature of efficient cause,” Aquinas begins. “There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible.” Imagine that you look outside your window and see a tree branch moving. That branch is being moved by the wind. That wind has its causes, and so on. But as with motions, you cannot go on to infinity in a series of causes. If there is no first cause, there will no be intermediate causes, like the wind and the swaying tree branches. “Therefore it is necessary to admit a first efficient cause, to which everyone else gives the name of God,” Aquinas concludes.

These first two arguments are cosmological, since they show features of the world order, like motions and causes, and claim that these realities require an explanation from a divine reality outside the world order.

Teleological Argument

Aquinas's fifth way, or argument, is a teleological argument for God's existence. It is sometimes referred to as his “design argument,” and it is his most renowned one. Stated in its entirety, it reads:

The fifth way is taken from the governance of the world. We see that things which lack knowledge, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that they achieve their end, not fortuitously, but designedly. Now whatever lacks knowledge cannot move toward an end, unless it is directed by some being endowed with knowledge and intelligence; as the arrow is directed by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

Here Aquinas, an empiricist, shows his “teleological” understanding of nature (the word teleology means “purposes, goals, or ends”). This religious view of the physical world says that nature acts as if it were following a purpose or aiming at some mark. Aquinas's analogy of the arrow being directed by the archer suggests intelligent design. How else could an arrow move toward its target unless someone directed it that way? Likewise, since nature exhibits regularity — as in the motions of planets, the regular succession of seasons, and so on — this regularity must have been caused by an intelligent designer.

Aquinas designed the five proofs not to satisfy the critical minds of mature philosophers, but as introductory material for “novices” in the study of theology. Still, one finds in his Summa Theologica (the “sum of theology”) that he speaks of proof, or demonstration. And by demonstration he means in this context what he calls demonstration quia, namely a causal proof of God's existence, proceeding from the affirmation of some empirical fact, for example that there are things that change, to the affirmation of a transcendent cause.

The proofs remind one of Aquinas's position on faith and reason. The realm of human knowledge can be divided into two areas:

• Truths known to us in revelation and known by faith

• Truths revealed in nature and known by reasoning from experience

His proofs of God's existence fit this last category. In so doing, he shows his allegiance to the empiricism of Aristotle. In Summa Theologica, quoting Aristotle, he says that prior to experience the mind is like a blank tablet. In contrast to Plato and his followers, Aquinas does not accept the idea that there is innate knowledge. Even the idea of God is not written on the mind. But sensory cognition is not enough to explain our knowledge. “The intellect knows many things which the senses cannot perceive.”