As a BARBARA syllogism, the First Way is:
Everything moved by another is moved by a first, immobile mover.
Everything in motion is moved by another.
Everything in motion is moved by a first, immobile mover.
Now if we step back from the controversial fact that this is supposed to be proving the existence of God (which, for whatever reason, always tends to trigger logical hyper-scrupulosity), we might take it as a rather uncontroversial proof for either laws of nature or something like energy.* Physics is in the business of explaining why things move around, and it always explains things in terms of some first thing that always remains unchanged in itself. If you see a bunch of things fall in the inverse square of the distance, you account for this by saying there is some law of gravity, and this law of gravity doesn’t move around – whether in the inverse square of the distance or in any other way. Again, if you see things moving around, you account for their motions in terms of energy or something like it, and this energy doesn’t ever change qua energy, but only manifests itself in different modes. So the major premise and the conclusion seem to be uncontroversial claims – which gives us the minor premise for free by logic alone (though this is usually the most controversial premise when the FW is usually presented)
But even if the laws of nature or something-like-energy are immobile in themselves, neither suffice to explain why anything is actually in motion. Laws of nature only give us actual motion if we help ourselves to initial conditions, since laws are ways of relating initial conditions to outcomes. Energy also doesn’t explain why something is actually moving, since energy is the ability to do work but a thing has the same ability to do work whether it is actually working or not. And so while physics explains things in motion by relating them to the immobile, the immobility it attains to is still not adequate to account for the things of experience, leaving us having to add something to physics in order to account for the very thing it’s trying to explain.
Read in this way, the FW is an apologia for metaphysics, that is, for a science distinct from but contiguous with physics which carries on the explanatory enterprise to its conclusion. Physics carries the explanation as far as it can go along the line of things that can be given to sensation, which allows at most for things that interact with something we can sense either directly or through an apparatus. The physicist’s job is done as soon as he hits on some unseen power that works interactively, that is, the power of an entity that cannot exist apart from something at least potentially sensible, like laws of nature, which are defined with physical and sensible things (like meters, masses, speeds, etc.).
*”Something like energy”, i.e. maybe mass-energy, or whatever might wind up unifying all the fundamental forces, or what might end up unifying the fundamental forces with dark energy, or what might unify the dark energy-fundamental forces thingamajigger with something else we end up discovering. At any point on this progression, energy is something remaining the same in itself and yet manifesting in diverse modes to give us diverse forces.