'Can' vs. 'may'

In the 1800s, using the word “can” to ask for permission was considered standard English. But in the century that followed, something happened. Grammar fussbudgets got it in their heads that “can” should refer to ability and “may” should refer to permission. So if you ask whether you can go to the bathroom, you’re not asking to be excused but instead asking about the state of your own digestive health.
Where did they get this idea? No one knows. But it could have to do with the fact that “may” was being used for permission centuries before “can” existed in our language. And if you go back far enough, you unearth a rather delicious irony: In the eighth century, “may” referred to ability — just as “can” does today. Theoretically, you could have said, “I’m so powerful that I may lift this boulder over my head.” Around the same time, “may” adopted a second meaning, possibility, which is still in use today: It may rain tomorrow. This is also when “may” started to refer to permission: May I please be excused?

About 200 years later, “can” showed up in the language. At that time, “can” didn’t refer to ability, exactly. It meant to know something or to know how to do something. So you could have said “I can do math,” but you couldn’t say “I can lift this boulder,” since boulder-hoisting doesn’t require know-how.
It took a few more centuries before “can” came to mean “to be able to do something” — at a time when “may” was already doing that job. And by the year 1500, “can” and “may” had overlapping meanings.
So for hundreds of years, “can” and “may” both meant to be able to do something or that something was possible. During that time, “may” also meant to have permission, while “can” did not. That changed around 1800 when “can” started being used to refer to permission, making “Can I go to the bathroom?” proper speech. Then came the pushback.

Here's my recent column on "can" vs. "may" and why the rules aren't as strict as your mother taught you.

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