“The greatest deception men suffer is from their own opinions.” – Leonardo da Vinci – Italian polymath
Personal judgement becomes dangerous when it is mistaken for evidence. An opinion can begin as a useful interpretation, yet repeated often enough it hardens into a private reality, filtering what a person notices, remembers and accepts. The resulting deception is rarely deliberate. It arises because the mind prefers coherence to correction, familiarity to uncertainty and conclusions that protect identity to observations that threaten it. That mechanism applies to private decisions, political judgement, scientific inquiry and organisational strategy alike.
Leonardo’s notebooks provide a revealing context for this concern. He approached nature through close observation, drawing, comparison and repeated investigation rather than relying solely on inherited authority. Surviving records show studies of anatomy, water, flight, geology, optics, machines and painting, with drawings functioning not merely as illustrations but as instruments for inspecting relationships and motion.1,2 His practice was not a neat modern scientific method: he moved between workshop knowledge, analogy, measurement, visual analysis and speculation. Yet the recurring discipline was clear. Claims had to encounter the material world.
That discipline matters because opinions often conceal their own starting assumptions. A person may believe that a market will recover, that a colleague is unreliable, that a technology cannot scale or that an established explanation must be correct. Once the belief becomes part of self-image, contrary evidence is processed as an inconvenience rather than as information. Confirmation bias then encourages selective attention, while motivated reasoning supplies plausible arguments for a conclusion chosen in advance. The person can remain intellectually active while becoming less capable of being corrected.
Observation against inherited certainty
Leonardo’s importance lies less in presenting a universal formula for truth than in modelling a posture towards uncertainty. He repeatedly returned to direct observation and urged the testing of general rules through multiple trials.3 Such habits do not eliminate error. Measurements can be poor, observations incomplete and interpretations shaped by prior expectations. They do, however, create friction against self-deception. A claim that must survive comparison, replication or contact with an unfamiliar case is less secure than one protected only by confidence.
The historical context also requires caution. Calling Leonardo a scientist in the modern professional sense risks imposing later institutions and standards on a Renaissance thinker. The specialised disciplines, journals and formal research communities associated with contemporary science did not yet exist.1 His investigations were ambitious but uneven, and many findings were never published or integrated into later scientific traditions. The Metropolitan Museum of Art describes his notebooks as records of persistent observation, experimentation and invention, while also noting the limited number of completed paintings and the central role of drawing in his investigations.8 His example is therefore strongest as a study in intellectual practice, not as evidence that he anticipated every feature of modern science.
Attribution presents a related problem. The wording is commonly listed among passages from the English editions of Leonardo’s notebooks, including the numbering associated with reflections on foolishness and ignorance.6 However, quotation databases and popular compilations are not equivalent to a critical edition of the original manuscripts. Translation can alter tone, grammar and emphasis, while editorial selection can detach a sentence from its surrounding notes. The underlying idea is consistent with Leonardo’s broader emphasis on experience and examination, but the exact English wording should be treated as a translated notebook passage rather than as a modern transcript of an authenticated spoken remark.
The strategic cost of being persuaded by oneself
In organisations, self-deception becomes expensive when authority suppresses disconfirming evidence. Leaders may interpret early success as proof that a strategy is sound, confuse loyalty with competence or treat a forecast as a commitment. Teams then optimise for agreement, because dissent appears to threaten cohesion. The result is not simply a mistaken opinion but a system that generates supporting data: targets are adjusted, exceptions are excluded and bad news travels slowly. By the time failure becomes undeniable, the organisation may have lost the time needed to respond.
The remedy is not to replace judgement with mechanical neutrality. Every investigation begins with questions, categories and provisional expectations. The practical safeguard is to make those assumptions visible and expose them to structured challenge. Decision-makers can record what they believe, what evidence would change their minds and which observations would count against the plan. Independent review, red-team analysis, post-decision audits and clearly separated evidence from interpretation can reduce the ability of confidence to disguise uncertainty.
Leonardo’s interdisciplinary work offers an additional lesson. He did not isolate painting from anatomy, engineering from geometry or natural observation from design. His studies of bodies, light, water and movement fed both artistic and technical thinking.2,8 Crossing domains can weaken entrenched opinions because an assumption that appears persuasive in one setting may look implausible in another. It can also create new errors if analogy is treated as proof. A model transferred from mechanics to society, or from biology to business, remains a hypothesis until the relevant evidence supports the transfer.
Why the warning remains relevant
Modern information systems intensify the problem. Personalised feeds, reputation incentives and rapid commentary allow people to inhabit environments that reward familiar conclusions. The volume of available information can create an illusion of investigation while narrowing the range of material actually considered. More data does not automatically produce better judgement if the selection process is governed by an unexamined opinion. The crucial question is not whether evidence exists, but whether the believer has built a credible route by which inconvenient evidence can be noticed and accepted.
The most constructive reading is therefore demanding rather than cynical. Doubt should not mean treating every claim as equally valid, and openness to correction does not require abandoning expertise. It means distinguishing confidence from certainty, inference from observation and a useful working model from reality itself. Leonardo’s notebooks remain valuable precisely because they show thought in motion: questions revised, forms redrawn, mechanisms reconsidered and explanations brought back to experience.1,10 The intellectual danger lies not in having opinions, but in allowing them to become immune to the world they are supposed to explain.
References
1. Leonardo and Science | TheHistoryOfArt.org – https://www.thehistoryofart.org/leonardo-da-vinci/science/
2. Leonardo Da Vinci – The Genius – Museum of Science – https://www.mos.org/leonardo/index.html
3. Leonardo da Vinci – Wikipedia – 2001-09-07 – https://en.wikipedia.org/wiki/Leonardo_Da_Vinci
4. www.newadvent.org › cathen › 15440aCATHOLIC ENCYCLOPEDIA: Leonardo da Vinci – NEW ADVENT – https://www.newadvent.org/cathen/15440a.htm
5. [PDF] Leonardo da Vinci – The Scientist – FUP Riviste – https://riviste.fupress.net/index.php/subs/article/download/636/277/1820
6. Leonardo da Vinci quote about opinion from The Notebooks of Leonardo Da Vinci – 2025-01-01 – https://kwize.com/quote/5166
7. Leonardo da Vinci | History | Research Starters – EBSCO – 2024-10-01 – https://www.ebsco.com/research-starters/history/leonardo-da-vinci
8. Leonardo da Vinci (1452-1519) – The Metropolitan Museum of Art – 2002-10-01 – https://www.metmuseum.org/essays/leonardo-da-vinci-1452-1519
9. An Interpreter of Nature – https://erenow.org/biographies/the-science-of-leonardo-inside-the-mind-of-the-great-genius-of-the-renaissance/2.php
10. Leonardo da Vinci’s Disciplines | Studies in Art, Science and Anatomy – 2024-11-04 – https://www.pbs.org/kenburns/leonardo-da-vinci/leonardo-da-vinci-disciplines
11. Scientist | Leonardo Da Vinci – The Genius – https://www.mos.org/leonardo/scientist.html
12. Leonardo da Vinci Quotes about men – https://libquotes.com/leonardo-da-vinci/quotes/men
13. www.thehistoryofart.org › leonardo-da-vinci › biographyLeonardo da Vinci Biography: Life, Art and Invention – https://www.thehistoryofart.org/leonardo-da-vinci/biography/
14. Leonardo da Vinci | Paintings, Art, Last Supper … – Britannica – 2026-06-09 – https://www.britannica.com/biography/Leonardo-da-Vinci
15. Leonardo da Vinci Quote – https://www.azquotes.com/quote/70619
