As some people interested in English history know, there was a movement a few hundred years ago to make English a logical language, in the sense of obeying the rules of predicate logic. One example of this is the attempted eradication of double negation. In symbolic logic, ~p means "it is not the case that p is true" regardless of what p is. It could be a whole series of statements, and the one negation negates them all. This was not true in English until the logicians made it so. Many people still use double negation, but now it's a marked variant, a non-standard dialect. Another example is the use of the nominative case for verbs of being, i.e., "It is I". Logically, the logicians said, the copula there ("is") represents "=", and thus the word preceding and following it should have the same case. It seems, however, that ultimately the logicians have lost.
The token that really brought this home to me is the Godspeed You! Black Emperor song "motherfucker = redeemer". I was puzzling over what the song title could mean, and realized that one reading that was certainly not possible was that every motherfucker is a redeemer and that ever redeemer is a motherfucker, which in logic and math is what the symbol "=" is used for. 2+4=6 is a truth, no matter how you look at it. However, using the equal sign in English generally means that the left hand item is equivalent to the right hand item, but not vice versa. The copula works the same way. If I say "Computational linguists are jerks", I don't mean that every jerk is a computational linguist, but I probably mean that every computational linguist is a jerk (I didn't say it was an accurate statement; it's just an example). Sorry, logic. Natural languages don't really like you.
Writer’s Craft # 121 Realistic dialogue
1 week ago