#nightmare

This was the one that made him, from what I've read — it caused a real stir when it first went on show, mostly because of how sexual people found it, and he painted the idea more than once afterwards. Much later people started treating it as an early go at the unconscious, decades before anyone had the vocabulary for that.

These were sold as a set of eighty, in an ordinary shop rather than anywhere grand, and pulled off sale after a couple of weeks — the usual explanation is that he got nervous about the Inquisition. This one was meant to open the second half, and there's a note attributed to him saying the idea is that reason and imagination go wrong without each other. Whether that's the whole story, I couldn't tell you.