Thomas Aquinas, Compendium of Theology

God cannot be the form of a body or any kind of force existing in a body. For, since all bodies are found to be mobile, whatever is present in a body must be moved, at least per accidens or concomitantly, if the body itself is moved. The first mover, however, cannot be moved either per se or per accidens, for it must be absolutely immobile, as has been shown. Therefore God cannot be a body or a force in a body.

Again, in order to move an object, every mover must have dominion over the thing that is moved. For we observe that motion is more rapid in proportion as the motive force exceeds the resisting force of the mobile object. Therefore that which is the very first among all movers, must predominate supremely over all the things moved. But this would be impossible if the mover were in any way attached to the mobile object, as it would have to be if it were the form or motive power of the latter. Consequently the first mover cannot be a body or a force in a body or a form in a body. This is why Anaxagoras postulated an intelligence liberated from matter, that it might rule and move all things.