Demonstrations can be made in two ways: one is through the cause, and is called propter quid, and this is to argue from what is prior absolutely. The other is through the effect, and this is called a demonstration quia; this is to argue from what is prior relatively only to us.Another translation, from the Fathers of the English Dominican Province:
Demonstration can be made in two ways: One is through the cause, and is called a priori, and this is to argue from what is prior absolutely. The other is through the effect, and is called a demonstration a posteriori; this is to argue from what is prior relatively only to us.Natural science, which argues not through the cause of the universe (i.e., with a demonstration propter quid or a priori), can only prove the existence of this Cause (i.e., God) through Its effects (i.e., with a demonstration quid or a posteriori), even if natural science, the study of the physical world, can never know the essence of God, Who is purely spiritual. Thus natural science, like physics, can demonstrate God's existence.
An atheist scientist is often a materialist; he maintains that matter is eternal and therefore does not need a creator. Science, especially quantum mechanics and particle physics, can prove that matter is not eternal; particles, such as matter–anti-matter pairs like an electron and anti-electron, can pop in and out of existence when interacting with each other, for example.
One must be careful about the meaning of the word "matter." When physicists say "matter," we mean "physical matter," but ὕλη in the philosophical sense is much broader. It does not just mean "that which has mass or, due to Einstein's equivalence of mass and energy E=mc², energy, too." From the Oxford English Dictionary:
This use of form (Aristotle's μορφή or εἶδος) and matter (ὕλη) is a metaphorical extension of their popular use. In ordinary speech, a portion of matter, stuff, or material, becomes a 'thing' by virtue of having a particular 'form' or shape; by altering the form, the matter remaining unchanged, we make a new 'thing'. This language, primarily applied only to objects of sense, was in philosophical use extended to objects of thought: every 'thing' or entity was viewed as consisting of two elements, its form by virtue of which it was different from, and its matter which it had in common with, others.Thus in the case of the "annihilating" electron and anti-electron, matter, even in the restricted physicists' sense of the word, is not reduced to nothing (i.e., annihilated). No, this matter, in the philosopher's sense, merely changes form; it was an electron and anti-electron before interacting, and now it is pure energy.
It would then seem that matter is eternal. But how can it be if natural science can demonstrate the existence of a Creator?