One of the small formative moments in my philosophical life came in a undergraduate course on Islamic philosophy. It was a one-off course for the department, the fruits of having been lucky to land a temporary lecturer who had a side interest in the topic. We were reading (I think) Muhammad ibn Zakariya al-Razi. In the text he was arguing that one of the proofs of the soul’s separation from the body was that while we could tire of physical action, we never tired of mental action.
To which youthful me immediately called bullshit. I assume, now, that al-Razi was making a subtler argument than I gave him credit for, attending to the fatigue of the body capable of shutting down our mental efforts, but even now I don’t really buy that mental actions don’t tire us. Mental action, even when the body is prime rested and wakeful shape, can wear you down.