The Difference between Both and Either

A quickie today, unless I find a suitable comic…

Both of these words generally apply to two entities, but they don’t quite mean the same thing. Here’s a good sentence describing the difference:

You may have either one, but not both.

Occasionally I read someone who was careless and get the words reversed.

Scientific American got it right on page 16 of the October 2016 issue:

Do we live in a universe where each discovery leads to deeper, more fundamental insights, or do we live in one where some parts have rhyme and reason, but others don’t? Dark matter offers either possibility.

You could plug “both possibilities” into that second sentence and it might feel okay because we don’t know dark matter’s role. But the author is deliberately making a subtle point: Dark matter leads to one or the other, but not both.

Remember that distinction.