From what I learned about Boltzmann's life during the past few days, I get the impression that he may have suffered from bipolar disorder. During a period of depression in 1906, he committed suicide.
Besides physics, Boltzmann was also interested in philosophy and gave a series of philosophy lectures in 1903 at the University of Vienna where he was working at that time. The journal Synthese devoted its April 1999 issue to Boltzmann. Included among the articles published in that issue was one consisting of non-mathematical excerpts from Boltzmann's preparatory notes for his philosophy lectures. The notes contain many questions (without answers) that Boltzmann was apparently asking himself, perhaps to stimulate his thinking. In one of the lecture notes,this is what he says about asking questions: What matters most is to ask the right question. It is evident that one and the same question can be asked in many different ways.
I may write more about Boltzmann in the future. For now, here are some questions from Boltzmann's lecture notes:
How does one express oneself in the most efficient way without wasting words? How is it done?
What does it mean to define? To define numbers?
Are numbers concrete or abstract?
Are Roman and Arabic numerals more or less arbitrary products of the human spirit?
Is philosophy crime, illness, or insanity?
Could all qualities be separated from a thing?
Does the external world disappear during sleep and death?
What does the equality of two quantities mean?
What does it mean that atoms exist?
What does possible mean?
Are causes transformations of energy?
Would another straw have broken the camel's back or a finger in the dike have saved Holland?
Is it only a slight thing to distinguish between slight things?