What philosophical ideas should non-philosophers know about?

Philosopher Iskra Fileva posed this nice question on FB, and I thought it would be interesting to pose it here for a wider audience.  Professor Fileva asked her friends to pick just "three top" ideas, but I'm happy if readers want to propose more than three.  From many years of teaching philosophy to law students (and very good law students I might add), as well as talking to very smart law faculty, here are a few familiar philosophical ideas that I think are valuable for non-philosophers to understand:  the difference between justification and truth, and, relatedly, the difference between an epistemic and a metaphysical claim; the idea of a "natural kind," whether understood in the Kripke-Putnam way, or the earlier Whewell-Mill-Quine way (I prefer the latter); the difference between realism and anti-realism about some domain, and its relationship to ideas of mind- or attitude-independence.   I'm sure there are many others, so welcome reader perspectives on this.  I'd be particularly interested to hear from non-philosophy readers about what philosophical ideas they have found especially useful.  It's also fine to answer this question with an eye to philosophical ideas "ordinary" people might find important in their lives, and not simply those that are helpful in other areas of scholarship or study.