What Makes a Substance Exist?

Socrates is human, and his humanity is not just a fancy accident. Prior to his being human, there's no him there at all. It makes him exist as a substance of the human cause. If Rusty ceases to be a cat, he just doesn't exist anymore. Most contemporary philosophers would say yes, but Aquinas would say no. So although accidents happen to the substance, the substance itself has a lot to say over which accidents happen to it.