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Philosophy of religion » ontological » Cosmological
According to Aquinas, the existence of God can be proved in five ways:
1. Motion
The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.2. The efficient cause
The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. 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. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God. 3. Possibility and necessity
The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be. But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence. Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing. Therefore, if at one time nothing was in existence, it would have been impossible for anything to have begun to exist; and thus even now nothing would be in existence - which is absurd. Therefore, not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has been already proved in regard to efficient causes. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.
In the first three of the five ways to God Aquinas, drawing on Aristotle, observes order in the universe (the word for order in Greek is Kosmos) and draws a logical conclusion about why that order exists. All cosmological arguments work in this way; they observe order, a principle or law in the universe and then conclude that there must be an explanation for this which is, as Aquinas puts it, what "all men speak of as God". Cosmological arguments may therefore be described as inductive, a posteriori and strong and like all such arguments, the best ways of refuting them is either to demonstrate that the order, law or principle is not present in the universe or to dispute that its cause is what "we call God".
Aquinas notes the fact that things move. He defined motion as "the reduction of something from potentiality to actuality"; motion is a change of state, not just obvious changes of objects in space. Clearly such changes do not happen spontaneously (think of Newton in physics). When they move have been moved by something else, and in turn move other things. Things don't just move without being moved, nor can they move without moving anything else (even if it is just air!) However, there cannot be an infinite regress of movers and movements; there must have been a first mover, a "prime mover" and this, concludes Aquinas, is "what all men speak of as God".
Consider a domino rally: focus in on a single domino, it is moved by the domino before it and that by the domino before that. Everybody knows there must be a first in the series and that must be knocked by a non-domino mover. Try this for a really impressive domino rally.
The second way
Aquinas also notes that all things have efficient causes, they don't just arise spontaneously but are caused and enabled to be the way they are by other things. Consider your own self; you are caused and enabled to be reading this today by your parents and secondarily by grandparents etc. Nothing can exist without at least one cause but there cannot be an infinite regress of causes. If nothing started the chain of causes and effects, we could not be here. There must be a first cause and this is "what everybody calls God." Gottfried Leibniz put forward another version of this argument, which hinged on his "principle of sufficient reason" in Theodicy (1710):
Suppose the book of the elements of geometry to have been eternal. One copy having been written down from an earlier one. It is evident that even though a reason can be given for the present book out of the past one, we should never come to a full reason. What is true of the books is also true of the states of the world. If you suppose the world eternal, you will suppose nothing but a succession of states, and will not find within them a sufficient reason.
Critics from David Hume in Dialogues concerning natural religion (1776) onwards have asked whether there is any need to presume a first cause - could not the series go on into infinity. Further, they have asked whether the whole has to share the characteristics of the parts. If each part of the universe is caused it doesn't mean that the universe as a whole isn't uncaused. In 1948 Bertrand Russell (the atheist analytic philosopher), in a famous radio-debate with Frederick Copleston SJ, a Jesuit Christian philosopher, said "Every man has a mother: and it seems to me that your argument is that therefore the human race has a mother, but obviously the human race hasn't a mother - that's a different logical sphere." However, as Copleston remarked, "You see, I don't believe in the infinity of a series of events (I mean a horizontal series so to speak) if such an infinity could be proved. If you add up chocolates, you get chocolates after all and not a sheep. If you add up chocolates to infinity, you presumably get an infinite number of chocolates. So if you add up contingent beings to infinity, you still get contingent beings, not a necessary being. An infinite series of contingent beings will be, to my way of thinking, as unable to cause itself as one contingent being.
The third way
Is much more interesting. Rather than conceiving of the universe as a linear series of movements and causes, Aquinas now looks at it as a web of things, each contingent on other things. Everything has the potential to be or not to be - everything seems to be contingent. Yet, if everything really did depend on other things, given infinite time and space, everything's potential not to be would be actualised simultaneously at some point. In this situation the universe would cease to be; nothing can come from nothing. Therefore in this case we must suppose the existence of something which is not contingent, whose existence depends on nothing outside itself, which cannot not exist, which could cause other contingent things to be actualised again. This is what everybody calls God. As Richard Swinburne concluded "It is extraordinary that there should exist anything at all. Surely the most natural state of affairs is simply nothing. But there is something. And so many things. Maybe chance could have thrown up the odd electron. But so many particles? Not everything will have an explanation. But the whole process of science and all other intellectual enquiry demands that we postulate the smallest number of brute facts. If we can explain the many bits of the universe by one simple being which keeps them in existence, we should do so, even if inevitably we cannot explain the existence of that simple being."
Each of Aquinas' three forms of the cosmological argument supposes the existence of a necessary being, a being which depends on nothing outside itself, which cannot-not-exist. Such a God is often said to have the property of aseity, from the Latin "to exist from and of itself". A prime mover, an uncaused causer, a being which is necessary by nature (de re). This necessary being is seen as the cause in fieri of the universe in the first and second ways. Like a match lighting a fire, it may or may not still be part of the fire. In way three the necessary being is the cause in esse of the universe, a sustaining force which underpins its continued existence. Nevertheless, critics of the argument question whether the concept of a necessary being is meaningful. Kant suggested that such a being is beyond our experience and thus beyond our knowing. We cannot prove the existence of God in this way, even if it may seem sensible to postulate the existence of God to explain the existence of the universe and its characteristics of order, goodness and so on.
Click through for Aquinas' fourth and fifth ways of proving the existence of God.