“Every man bears the whole form of the human condition.”Michel de Montaigne · Eseuri · 1580 · III, 2
What I condemn in another is a living possibility in me.
Montaigne claims there is no such thing as a stranger: each of us contains the potential for every human experience, from glory to infamy. The mechanism is recognition: what we condemn in others exists in us as a possibility, so studying oneself becomes studying everyone. When you rage at a friend's betrayal, you can recall having lied yourself; the pride of not being "like him" is a comforting illusion.
If every person contains the whole human range, then knowing others becomes a way of reading your own mirror. Montaigne does not propose tolerance out of politeness, but out of anatomy: the person you despise shows you a room in your own house that you have shut. Think of a colleague who constantly brags: your irritation hides your own hunger for recognition, which you feed in other ways. Seeing this does not make you identical to him, but it forbids you to look down on him. The next step is finding what you actually do with this recognition: how to judge without condemning.
Why it matters An antidote to hasty judgment of others and to the illusion that we are essentially different from those we criticize.