Question: “Dear Grammar Guy: I’ve got a question for you. I’ve always thought that ‘assume’ and ‘presume’ could be used interchangeably, but recently a friend told me that isn’t true. What’s the deal? When should I use one and not the other?”

Answer: Yes and no, dear reader. Yes and no. I’ll let the folks over at the Oxford Dictionaries set this one up:

“In common usage, both ‘assume’ and ‘presume’ can mean ‘suppose,’ and are often interchangeable in this meaning. However, technically there is a subtle difference between the two where ‘presume’ is to ‘suppose to be the case on the basis of probability.’ ‘Assume’ on the other hand is to ‘suppose to be the case without proof.’ – (The Oxford Dictionaries)

Catch that? ‘Presume’ should be used for a supposition based on probability or evidence; ‘assume’ should be used for one without.

How about some examples:

• “After John was indicted for perjury, many presumed his legal career was over.”
• “Before I tried spinach, I just assumed I wouldn’t like it.”

“Assume” and “presume” share a similar secondary meaning as well: “to take on oneself.” Oxford again differentiates the two, saying that “‘assume’ is generally used to describe taking on a role” while “‘presume’ is generally used when taking on an attitude.” I will add to Oxford’s description that “presume” is typically used in this sense as a negative, denoting arrogance or impertinence: “Don’t presume to tell me what to do.”

Thanks for writing in. Keep those questions coming!