Leonardo’s remark suggests that people are often misled not chiefly by others, but by their own beliefs and interpretations. Once an opinion is formed, it can shape how evidence is noticed, understood or dismissed, making it harder to recognise mistakes in one’s own judgement.

The statement therefore concerns the limits of personal certainty. Opinions may feel convincing because they are familiar or emotionally persuasive, yet confidence alone does not make them accurate. Careful reasoning requires a willingness to examine assumptions, compare them with evidence and consider alternative explanations.

The quote can be applied to debate, decision-making and everyday judgement. It encourages intellectual caution, especially when a belief seems obvious or deeply held. Its central point is that self-deception can arise when personal opinion is treated as fact without sufficient scrutiny.