Previous to that I had been meditating a lot on what it is like to be a rock.
No sense of time, no memory, no input at all. For all the rock knows, time would run in reverse, or jump all over the place. Obviously, the rock doesn't know about space either. Now if you are able to think up a universe without animals or humans, this opens up a lot of room to doubt some basic beliefs.
I recently found out that someone [1] ridiculed the philosophy on "what it's like to be" by using a rock as well. I was slightly disappointed by this, because the rock metaphor works fine for me. Perhaps I misunderstood the article.
[1] https://www.cs.bham.ac.uk/research/projects/cogaff/misc/like...