Stephen King believes that the writer's notebook is an excellent way to make bad ideas permanent. I tend to agree with him.

A good idea will stick with you. It will stick with you because it resonates with something deeper than whatever neurons participate in your short term memory. It will stick with you because it speaks to you, demands your attention, and refuses to go away—all of which trauma it will also (hopefully) inflict on your reader when the time comes. For this reason, it is not valuable to write down every stupid thought that enters your head. A notebook is mostly a way for bad ideas to survive the culling of time.

That said, I do write down ideas. I write down plans for stories. I write them down as a way to explore them, in some small way, and then it seems I forget about them. I was just going through my email and found a message sent by me to myself from early 2014. It contained links to Wikipedia articles on the trifunctional hypothesis and the triple goddess: the idea of maiden, mother, and crone, one of the seminal themes for the novel I've been working on for the past six months.

The email had never been opened, of course.