Descartes says that for an effect to occur, it must have a cause, and that cause must be at least, if not more, perfect than the effect it produces. In the physical world, we might use the term "entropy", or perhaps "conservation of momentum" to describe the same phenomena: for example, the effect of my throwing a stone is that the stone is imparted with a certain amount of energy, which will be no greater (and probably less) that the energy I expended in moving my arm. In building a bridge, all I am doing is rearranging bits of stuff that were there before.

It is difficult to find examples in the physical world where an effect has more "perfection" than its cause. Perhaps we could use the example of life itself: if evolutionary theory is true, then we must accept that sophisticated forms of life have come about from primitive ingredients.

But even if we grant that the "causal reality principle" is valid in the material world, it does not necessarily follow that we can assume it works in the case of ideas. Descartes himself believes that corporeal substance is fundamentally distinct and different from thinking substance, and so some stronger argument is required than just to say "it's like that in the physical world, so the world of ideas must be analogous".

Descartes says that our ideas of things are less perfect than the things themselves, although they are caused by them. So the idea I have of a stone is caused by my experience of the stone, but is in itself not so perfect as the stone itself. Given that I have the idea of a God is "infinite, eternal, immutable...", where can that idea have come from, if not from God Himself?

It is difficult to argue against this reasoning, because any counter example of an idea without a cause which we can produce can be dismissed as something which isn't perfect, and which is therefore just an amalgamation of other ideas. And by definition, I can't use as an example something which I can't conceive of, because as soon as I use it, I'd need some kind of conception of it.