Leonardo compares the intellect with physical things that deteriorate when left unused. Iron rusts, while stagnant water becomes foul or freezes; in the same way, he suggests that intellectual abilities weaken when they are not regularly exercised. Thinking, studying and solving problems are presented as activities that help preserve mental capacity.

The comparison emphasises continued practice rather than the passive possession of knowledge. Skills in reasoning, memory and judgement can become less effective if they are neglected, while regular use helps keep them active. The statement therefore treats intellectual development as an ongoing process rather than something permanently secured once knowledge has been acquired.

The quote can be applied to education and lifelong learning. It encourages continued reading, reflection, problem-solving and other forms of mental activity as ways of maintaining intellectual habits and preventing knowledge or skills from becoming neglected through disuse.