"[W]henever a man thinks about something that distresses him, and worry, grief, or sadness crop up in him, it can be due only to one of two things: either he is thinking about a matter that has already taken place, like someone who thinks about something that happened to him, such as the loss of his money, or about the death of someone dear to him; or else he is thinking about matters he expects and whose advent he dreads, like someone who thinks and talks about the advent of any disaster he expects."
- Maimonides