This is the question to dwell on I think. I don't know if there are mental ergonomics that apply universally, or even ones that generalise well. For me, I recognise that I think well when I'm physically engaged in some other non-cognitive task, like walking, running, showering, etc.
Another key factor is eliminating the risk of interruptions. I know when I'm at my desk I'm likely to get interrupted, but times and spaces that are interruption free seem to encourage the deepest thought.
Another key factor is eliminating the risk of interruptions. I know when I'm at my desk I'm likely to get interrupted, but times and spaces that are interruption free seem to encourage the deepest thought.