Ask HN: Calling all cynics what's your most usefully pessimistic heuristic?

was taking a stroll through paul graham's website and on his quotes page I read this:

"Politics is not the art of the possible. It consists in choosing between the disastrous and the unpalatable." - J. K. Galbraith, Letter to Kennedy, 1962

So, I wanted to ask all you out there: what's the most depressing quote(s) about society and the nature of mankind (as nebulous or granular a connotation of either word as you prefer) that has been useful to you as a heuristic for keeping your mental guard up and being seen how you want to be seen (or at least some recognizable semblance of how you want to be seen) in work and/or social circles?