For me, knowledge is fundamental: it gives me profound happiness, and I don't think about human misery. You can study anything; the important thing is to study. I see many elderly people waiting for death, no longer living life. Regardless of physical ailments, if the brain isn't affected, my advice is to always use it: it doesn't matter how small the topics are, the important thing is to always keep it functioning. In my opinion, studying is the best way to keep it active: it combines the useful with the pleasurable.