Talk:substitute

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Can someone please specify the correct usage of the word "substitute" (verb)?

For example, "You can substitute X for Y."?

Dictionary definitions that I have seen do not make it obvious, and I think I've heard its use in casual conversation both ways. I'm pretty sure that correct usage in the above example has the following sense: "You can use X instead of Y.", but can anyone provide a authoritative source that is explicit about this?

TIA

You are correct, "to substitute X for Y" means that X is the new, Y is the old. A synonym would be "to replace Y with X". The phrase "substitute X for Y" cannot have the opposite meaning, and I think the principle factor is the word "for". If you understand computer logic, you could rephrase it this way, where "for" = "if exists": if exists Y, substitute X. The preposition "for" does not admit the inverse sense, unlike Spanish and other Romance languages. —Stephen 06:49, 7 June 2006 (UTC)
Unfortunately, both meanings are used: "substitute Y with X". When "with X" is omitted the result is ambiguous, and the possible effects can be horrendous to contemplate. I've added a cautionary note. --Thnidu (talk) 23:50, 10 December 2014 (UTC)