It’s hard to tell good ideas apart from bad ones when they first burst into our mind.
Good ideas are rarely initially viewed as such, and bad ideas are too easily mistaken for worthwhile ones. So we either feel conflicted, like our ideas aren’t worth more than writing down (if that), or we feel certain that the ideas that are really worth anything will simply resurface at some point in the future.
But the occurrence of an idea isn’t any indication for how valuable it can be. The only way to determine whether an idea is good or bad is to explore it, either by intentionally ruminating on it or moving forward with it.
But in any case the best thing to do when an idea strikes is to get it out of your head, to write it down. That’s the only way you’ll be certain to come back to it, or evolve it, or remind yourself of what you once thought was possible.