Thank you for the very thoughtful comment!

I'm indeed not an English native speaker, so I wasn't fully aware of the different possible meanings of 'nothing in mind', but you're right. The German stamp has exactly the same ambiguity, which I thankfully took as a way out of the impossible situation of 'thinking nothing.

It's also very notable that the back of the paintings receives way more attention than the paintings themselves – but I guess that's what HN is all about (and what I like about it!), pondering the intellectual and philosophical problems of not thinking ;-)