In a 1921 entry for his newspaper column “How Do You Say It?: Common Errors in English and How to Avoid Them,” Charles N. Lurie laid out the difference between the auxiliary verbs can and may. The former, he explained, “means to be able to do or have the power of doing something,” while the latter “expresses permission or probability.” He then offered an example similar to what you might have seen during your own early school days:
“Thus, the pupil may incorrectly ask of the teacher, ‘Can I speak to my seatmate?’ and the teacher may reply, ‘Yes, you can speak to him,’ (meaning that the questioner has the power or the ability to do so), ‘but you may not do so,’ (meaning that the teacher’s permission is withheld).”
To Lurie, “Can I … ?” wasn’t just an improper way to request permission—it was grammatically incorrect. And though it has become much more acceptable in the last century or so, many people still think that can specifically refers to ability and may refers to authorization. Based on 20th-century etiquette, that’s not wrong. But if you assumed there’s some etymological history that proves the point, we have some surprising news for you.
According to Merriam-Webster, the first written mentions of may, dating back to the 8th century, actually had to do with power or ability. In Beowulf, for instance, the titular hero uses an early version of may (mæge) while declaring that he won’t kill Grendel with a sword even though it’s in his power to do so. (The line has also been translated as “though well I am able.”) May meaning “permission” or “probability” came later.
When can arrived around the 11th century, it didn’t refer to permission or ability; it meant “to know.” By the time that definition evolved into “to be able to” during the 14th century, people had already been using may in that sense for hundreds of years. The words later became synonyms in situations relating to probability, too; and people finally began co-opting may’s “to be allowed to” definition for can in the late 19th century.
Since can-as-permission was still relatively new in the early 20th century—and may-as-permission wasn’t—it’s not surprising that the era’s grammar sticklers felt that “Can I speak to my seatmate?” and similar statements were just wrong. But if correct word usage was always dictated by whatever definition is oldest, we should only be using may to talk about power and can to talk about knowledge.
These days, it’s more common to use may for probability and can for ability. When it comes to asking permission, however, people will likely understand your meaning either way. And if you get hit with the old “I don’t know, can you?” feel free to send the cheeky offender this article in response.