I often hear opponents to Thomas' First Way claim that one of the two central premises of the argument has been falsified by Newton's Laws. This is basically the entire First Way:
1. Evident to the senses is motion.
2. Everything in motion is moved by another.
3. If there is no First Mover, there is no motion.
4. Therefore, a First Mover exists. (1, 3)
I have already dedicated quite some time in support of (3). It is (2) that is alleged to have been shown false. I think such a conclusion vastly misunderstands the nature of Newton's Laws and, perhaps more importantly, what it is that Thomas is actually stating.
Newton's Laws tell us that if something is in motion, it will continue to be in motion so long as it travels in a straight line. Whatever continues this line of motion does not rely on something to keep its motion going. This should already provide sufficient warrant to raise a red flag. For, the motion of the celestial bodies (and earth) are not at all linear, but elliptical. From this it follows that we do not have any defeater for the premise that everything in motion is moved by another.
Moreover, even something that moves in a straight line without another thing moving it can only do so in the absence of a net external force - something like an absolute vacuum. But of course, such a vacuum does not exist. What we often associate with empty space in a vacuum is simply a convenient way to describe any space whose gaseous pressure is less than atmospheric pressure. The vacuum literally is *something*, rather than nothing, so it counts as a net external force. Newton's first law, while very helpful, is really just an approximation to the truth about motion. We still have to have a First Mover.
Imagine a watch that exists from all eternity. Even if there are infinitely-many gears, none of them will move apart from the cause of motion found in the watch's spring. If the spring is removed, then the gears of the watch will cease to move. By analogy, we can imagine the motion of the celestial bodies existing from all eternity. Even if this were so (barring any argument that the world is finite in age), the motion of these bodies is reducible to the motion caused by the First Mover. Without a First Mover, there can be no intermediate movers. Given that there are intermediate movers, it follows that a First Mover exists.
Thomas' First Way, however, leads only to a metaphysically necessary First Mover, and not to a logically necessary one. For, there are possible worlds in which no motion exists at all. Does the First Mover exist in these worlds, as well? One benefit of modalizing these arguments is that they very often get us to see that some entity actually is logically necessary. Consider the Modal First Way:
1. There possibly exists a First Mover. (In some possible world, a First Mover exists).
2. Whatever is possible is either contingent or necessary.
3. Whatever is contingent can be actualized.
4. A First Mover cannot be actualized.
5. Therefore, a First Mover exists necessarily.
It is often difficult for the atheist to decide which premise he/she actually disagrees with, but the argument is logically valid, so given the truth of each of the premises, the conclusion (5) necessarily follows. I actually went through this argument step-by-step in an earlier post.
I find the above proofs to be both rationally compelling and extraordinarily intriguing. What is left now is to deal with the Gap Problem: how do we get from First Mover to God?