Man's mind, once stretched by a new idea, never regains its original dimensions.

The aphorism frames learning as an irreversible expansion of consciousness: once a person genuinely grasps a new idea, the mind cannot return to its prior, narrower state. It suggests that insight changes not only what one knows but how one perceives—altering assumptions, categories, and the range of possibilities one can entertain. The image of “stretching” implies both growth and discomfort: new ideas can strain familiar habits of thought, yet that strain is the condition of intellectual development. In a legal or civic register (often associated with Holmes Jr.), it can also be read as a warning that exposure to new arguments or realities permanently complicates judgment and makes simplistic certainty harder to sustain.