If it's incorrect to "learn" someone, then why is "learned man" correct?

I am well aware that "learn" is incorrect when used as "teach" (referenced in Is 'learn' the new 'teach'?). So why is "learned" common fare, since it is apparently just a participial version of the incorrect usage?

^{Chart from Google Ngrams}

Solution 1:

According to etymonline

The transitive sense (He learned me how to read), now vulgar, was acceptable from c.1200 until early 19c., from Old English læran "to teach" (cognates: Dutch leren, German lehren "to teach," literally "to make known;" see lore), and is preserved in past participle adjective learned "having knowledge gained by study."

Solution 2:

It's worth considering why people developed the idea that learn for teach is incorrect.

As detailed in some of the answers you link to, the word learn originally had two fully-accepted meanings; to acquire knowledge and skills (as it is still uncontroversially used today) and to impart knowledge and skills (a synonym for teach).

Indeed, learn is in fact an etymological merging of one word derived from leornian (acquire knowledge) and another derived from læran (impart knowledge). As such, they should perhaps not be thought of as the same word at all, but as two homonyms. Notably the obsolete (perhaps remaining in some dialects) variant lere is a variant only of the sense meaning "to teach".

At this time we could of course use the participle learned to mean someone who had been taught a lot, because it was uncontroversial to use learn to mean teach. Therefore someone who had been taught was learned.

This participle use came to mean not just that the person had been taught something, but that they had been taught a lot. It also came to include auto-didacts. As it acquired this nuance that went beyond the meaning inherent in the use of the participle as a modifier it became an adjective separate to its verb origin; "A learned man" meant something that went beyond "A man who has been taught".

Now, it's worth considering why learn became considered incorrect relatively recently.

It's hard to be sure this is what was at play here (I don't know of any work arguing the case), but one trait that recurred among the prescriptivists of the 18th and 19th century was a desire for logic and well-ordered coordination between words with little overlap.

Take for example the words less and fewer:

Fewer can only be applied to countable nouns; I can have fewer peas but I can't pour fewer sauce on them. Less can be applied to both countable and uncountable nouns; I can have less peas and pour less sauce on them. This irritated some people and so less was beaten back to only deal with uncountable nouns. (In fairness, it was stated in 1770 as a preference rather than a rule, and it's a perfectly reasonable preference).

Most nouns and verbs have a grammatical number; a given form is either singular or plural. None is both singular and plural. This irritated some people and so none was beaten back to only be singular. (I don't actually have a source for this being given as a rule, but I do know of Archbishop Lowth arguing against it, which suggests it was around that time that it came into being).

It seems a reasonable conjecture to suppose that a similar preference was at play here: Learn can mean both acquiring and imparting knowledge while teach can mean only imparting knowledge, and so learn was beaten back to only mean acquiring knowledge. (Even though strictly those were two separate homonyms).

This would not have applied to the adjectival use: Learned means having a lot of knowledge and… and that's it. There isn't a rival. There isn't an overlap. Perhaps if we also had the expression "a taught man" meaning the same thing, then learned in this adjectival use would also have come to be considered incorrect, but we didn't, so it wasn't. (We do have "well-taught", but the slightly different form and slightly different meaning ["well-taught" excludes the autodidact while learned now does not] means there isn't a direct analogy).

The fact that learned as an adjective already implied more than just the consideration of its meaning as a predicate would suggest no doubt helped insulate it from this culling of senses from learn. Afterwards the declined of that sense of learn would have only further strengthened the independence of the adjective.

Solution 3:

It is very likely that the n in learned is a "correction" that shouldn't have happened. In the Speculum Vitae, among other Middle English sources, we find this form:

Bothe lered and lewed, olde and yonge,

Alle vnderstonden english tonge.

Lewed has since become lewd, and like its friends rude and homely, has acquired a connotation that wasn't there to begin with. At the time, it simply meant uneducated or unlettered, the opposite of lered.

We have managed to keep the pronounced e in the ending (and sometimes mark it with a grave accent in spelling, as learnèd) that marks the word as an adjective rather than the past tense of a verb (and have done so more strongly than in other similar words); the n, which used to mark the verb form, likely came in because people crave consistency to the point that they make their languages inconsistent to get it.