When people will not weed their own minds, they are apt to be overrun with nettles.

Walpole’s metaphor treats the mind as a garden requiring regular cultivation. “Weeding” stands for self-scrutiny, education, and the deliberate correction of bad habits or prejudices. If a person neglects this inner maintenance, the mind becomes “overrun with nettles”—painful, invasive growths suggesting ignorance, superstition, resentment, or unexamined opinions that spread quickly and are hard to uproot. The aphorism reflects an Enlightenment-inflected moral psychology: character and judgment are not fixed but formed through attention and discipline. It also implies a social warning—uncultivated minds are vulnerable to fashionable nonsense and manipulation.