Experience is the mistress of fools.

Here “mistress” has the older sense of a woman teacher or master, not its later sexual or extramarital sense. The saying says that a fool is taught chiefly by personal mistakes and their consequences, rather than by foresight, advice, or others’ examples.

It contrasts costly trial and error with wiser learning. Experience can teach anyone, but relying on it alone may mean learning only after harm has occurred.

Use it as a caution against ignoring sound advice or obvious lessons until direct consequences force recognition. It suits reflective or admonitory writing about repeated, avoidable mistakes, but can sound contemptuous when directed at a person and should be used carefully in personal criticism.