. Where there is a being in change, i.e., passing from potency to actuality, there must be another being actually existent, outside the series in change, whether this series is considered to be finite or infinite.Aquinas formulates this principle in three different ways according to the three aspects of reality taken into consideration. For the first way the formulation is: What is moved, is moved by another; for the second way: It is impossible for something to be the efficient cause of itself; for the third way: What is not, cannot begin to be, unless by force of something which is.The fourth way takes into consideration many aspects of reality, which, when compared with one another, show that they are more or less perfect. The principle of intelligibility is the following: What is said to be the greatest in any order of perfection is also the cause of all that exists in that order.The fifth way takes into consideration the order of nature: Where there is a tendency of many to the same end, there must be an intellectual being causing such an order.Let us set forth the schematic structure of the five ways:(1) Our senses attest to the existence of movement or motion. But every motion presupposes a mover which produces that movement. To have recourse to an infinite series of motions is not possible, for such an infinite series does not and cannot solve the question of the origin of the movement. Hence there exists a first mover that moves and is not itself moved. This is God. (2) Some new thing is produced. But every new production includes the concept of cause. Thus there exists a first cause which is itself not caused. This is God. (3) Everything in the world is contingent; that is, it may or may not exist. We know from experience that all things change in one way or another. But that which is contingent does not have the reason of its existence in itself, but in another, that is, in something which is not contingent. Hence there exist...
