Everyone seems to have an answer to this question. Here's mine:

Good ideas come from the expansive meadows of the mind where multitudes of bad ideas have just as much freedom to run around as the good ones.

Your brain is always trying its hardest. If you dismiss all bad ideas at the outset, then your brain learns its lesson and stops speaking up. You're biting the hand that's trying to feed you.

Ideate now, filter later.