^ I'll have to have a rummage and see if I can get back to you on that. I did a quick search, but it didn't turn up much other than references to my own forum posts - the search is probably a bit literal though, as I think I paraphrased from something much longer.
I have a vague memory of it coming from something I read on a blog when I was first researching my diagnosis, though I don't recall if that was the original source either (I think probably not, as I don't like to blatantly rob such things without giving credit - which of course, I will gladly do if anybody else finds the source in the mean-time.)
I do love it as a metaphor for dealing with "invisible" disabilities. It also reminds me of the monster from one of my favourite films, Forbidden Planet, which came into being invisibly from the central character's psyche, inadvertently affecting the lives of everyone around him (itself a sci-fi re-telling of Shakespeare's The Tempest.)
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When you are fighting an invisible monster, first throw a bucket of paint over it.