We often discover what will do, by finding out what will not do; and probably he who never made a mistake never made a discovery.

Smiles frames error as an indispensable instrument of progress. The first clause treats failure as negative knowledge: by eliminating what “will not do,” we narrow the field toward workable solutions. The second clause generalizes this into a maxim about discovery: genuine innovation requires risk, trial, and the willingness to be wrong. In Smiles’s characteristic self-help moralism, mistakes are not merely tolerable but productive, provided they are met with reflection and perseverance. The aphorism also implicitly criticizes excessive caution and perfectionism, suggesting that a spotless record may indicate not excellence but a lack of experimentation.