Ultracrepidarianism

If you don’t understand the basics of a subject, it’s easy to form conclusions that seem logical, but these same conclusions seem silly to those who have a deeper understanding of a subject. Ultracrepidarianism is the tendency for people to confidently make authoritative pronouncements in matters above or outside one's level of knowledge. Often, those pronouncements fall entirely outside the ultracrepidarian's realm of legitimate expertise.

Another expression of ultracrepidarianism, as instantiated by those with an actual expertise in something, is the tendency to start treating all other fields as somehow being sub-categories to your own field: epistemologists saying "it's all epistemology in the end", mathematicians saying "it's all mathematics in the end", physicists saying "it's all physics in the end", psychologists saying "it's all psychology in the end" (et cetera) and thus proceeding to apply their methods to a completely different field which they hardly realize they don't understand.

The lesson is that being an expert means being an expert at something — and "something" is specific, not universal. In other words, various forms of expertise are not interchangeable.

Explanation
Don't assume that because somebody has one intellectual skillset, they have another — that those tools apply to all types of intelligence, thinking or claims. They don't. If the quoted "authority" has no expertise in the relevant field, then their authority is irrelevant to the truth of the matter.

For example, a claim that "the speed of light is about 299,792,458 meters per second" could be supported with the statements of physicists who've studied the issue; it could not be supported with the statements of a manufacturer of crayons, or even those of a biologist or chemist (that is, persons who are legitimate scientists, but in a different field). Of course, this is a gross simplification. The value of the speed of light is derived from the definition of the meter, established (most recently) in 1983 by the 17th General Conference of Weights and Measures. Its value is generally-accepted basic scientific knowledge, an easily findable and learnable fact; even the most bloody-minded, hostile, and mean-drunk physics professor imaginable is not going to ding a Ph.D. candidate for not citing a source for it. Anyone who doesn't feel the need to reinvent the wheel by measuring the speed of light for him/herself can look it up in a standard reference — or even by googling "speed of light".

Stating it requires no particular expertise. Strictly speaking, Bill Nye, in stating the value of the speed of light, is resting the statement on the authority of the 17th General Conference just as much as the average microbial geneticist or gherkin importer would be, and either one could and would refer a person who questions the value to any standard reference.

The important point here is that the speed of light is so well researched, demonstrated, and accepted that its value is non-controversial. Ultracrepidarianism would enter the picture if a microbial geneticist (or a gherkin importer), rather than someone in a relevant field of physics, were to claim that the currently-accepted value for the speed of light is meaningfully incorrect. Or, of course, if a physicist were to challenge work in sociology (unless perhaps the challenge was based on faulty statistics, as both physicists and sociologists make great use of statistics in their work).

Origin
Pliny the Elder records the Greek artist Apelles' willingness to accept a cobbler's criticism of his rendering of a sandal, but rejection of other artistic advice, saying, "sutor, ne ultra crepidam", or, "shoemaker, not above the sandal".

A plain English adage, "cobbler, stick to your last", means essentially the same thing; last, in this case, is the mold a shoe is formed around.

Examples
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