The claim is that instruction works through demonstration rather than through telling. Children and pupils observe how the people around them actually behave, and take their cues from that, so conduct communicates far more reliably than advice does. Anything said that is contradicted by behaviour tends to be discounted.

The second half turns the point into a joke with a serious edge. Nobody can opt out of being observed, so the choice is not whether to set an example but which kind. Someone whose behaviour falls short still teaches, showing what to avoid rather than what to imitate.

Presenting this as the only rational method is deliberately absolute, and few would argue that direct instruction has no place at all. The narrower point, that behaviour carries more weight than precept, is widely accepted in accounts of how children learn.