I read a sentence:
It is not fortitude to be brave from ignorance and folly.
Why we use from in the construction brave from?
What is the full meaning of the sentence?
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It means "...to be brave because you are ignorant and foolish".
A person who goes into a dangerous situation even though they are fully aware of the risks is more truly brave than one who rushes in without stopping to think.
I don't think from should be used here, rather out of.
Here is an example in an article heading: Objective knowledge out of ignorance: Popper on body, mind, and the third world
Another one: Probability magic or knowledge out of ignorance
from is a funny little word undervalued in English in favor of of.
By my estimate the -m marks the case, which had come to supplement the ablativ and vocative in Germanic tongues. Compare whom, him; German dative von wem instead of genitive wessen, ca. "of whom" and "whose"; Latin accusative quam, "in what way".
This is notable because pro or fro (as in "to and fro, back and forth") are usually directed at the target, cp. e.g. in front of. Also cp. Latin per "via", Fr. par excelence.
It still makes sense however to read pro, only in reversed direction. I acknowledge that that is not really a satisfying conclusion.
Nevertheless, it is not too far off the mark, if en.wiktionary is correctly giving a reconstructed sense "2. (used with dative) by, due to" for *fram.