The XIII century saw a big controversy on whether it could be proved, and not merely accepted by faith, that the world began. 

1. All the classical Greek and Roman philosophers were in agreement that the world is eternal in the sense that it has always existed and will always exist. Aquinas, like all orthodox Christian philosophers disagreed because scripture teaches us otherwise. Hence, he set out to show that the arguments for the eternity of the world are inconclusive. Here are a few of them.

• All generation and corruption presuppose a material substrate. For example, the generation of a dog is always from some some matter provided by its parents; similarly, all corruption is always into something: when the dog dies it will become a corpse, and eventually dust. Hence, matter is eternal.

  Reply: the production of the world is not a case of generation but of creation, which is not a physical, natural phenomenon but a supernatural one.

• Time is coeval with the world. Hence, if the world eagan, there must have been a first moment of time. But this cannot be because it is the nature of time to join past and future (this argument is from Aristotle).

  Reply: It is true that once time has begun it joins past and future, but this does not apply to the first moment of time.

• Motion is always caused by a previous motion. Hence, there cannot have been a first motion, that is, motion must be eternal. Consequently, what is in motion must be eternal as well (this argument is also from Aristotle).

  Reply: Creation does not involve any sort of motion because creation is not any sort of change. In addition, God does not change in creating the world.

So, it cannot be proved that the world is eternal. However, this leaves open the issue of whether it can be proved that the world is not eternal. For example, Saint Bonaventure held that it can be proved that the world began. Saint Thomas Aquinas disagreed. Here we look at some of the arguments of Saint Thomas.
2. Since Aquinas adopted the Aristotelean theory of demonstration, according to which science deals with what is necessary (that which cannot be otherwise), he held that it's impossible to prove that the world began because:

• The essence of things is independent of any temporal modality.  For example, it's impossible to derive from the essence of man whether man is sempiternal or not.
• The divine will concerning creation is not necessary (e.g. God didn't create out of necessity, nor was it necessary that God would create this world instead of a different one).

3. Aquinas considers eight objections to his view (that is, eight arguments allegedly proving that the world is not eternal, i.e., that the world began), and answers them.  Here we'll look only at five:

1. One can prove that the world is caused (created) by God. But an effect must have a beginning in time, and therefore the world is not eternal.

   Reply: God's act of creation is not successive but instantaneous, and an effect of an instantaneous act need not have a beginning. Augustine's example of the eternal foot imprint which is eternal because its maker (the fooot) is also eternal.

2. Nothing can be equal to God in any respect.  But if the world had no beginning, it would be equal to God with respect to infinite duration.  Hence, the world has a beginning.
   Reply: Divine duration is not successive. (This ivolves the distinction between eternity proper and sempiternity).
3. If an infinity of days had to pass before today, then today would never had arrived because it's impossible to traverse the infinite.
   Reply: this presupposes a starting day by adding to which we get to today. But there is no such starting day, and from any day in the past one can get to the present one in a finite number of steps.
4. If the world were eternal, then any man would have been begotten of a previous one in an infinite series.  But the father is the efficient cause of the son, and an infinity of efficient causes is impossible.

   Reply: Aquinas draws a distinction between two types of series of efficient causes:

   • efficient causes which are required per se to bring about a certain effect (think of a big clockwork in which cogwheel B can move cogwheel C only insofar as it's being moved by cogwheel A).  In this case, an infinity of causes is impossible (there must be a spring moving the first cogwheel)
   • efficient causes which are not required per se to bring about a certain effect. A man B generates another man C not as a son of a previous man A, but as a man (A's generating power needn't be around for B to generate C).

 

5. If the world were eternal, then there would be an infinity of souls of human beings. But no infinite number of anything can exists.

   Reply: One could reply that the soul dies with the body, or appeal to reincarnation, or deny that human souls always existed.