‘Assume’ and ‘presume’ are often used interchangeably, but they have distinct meanings. So although both are related to the word ‘suppose’, it’s important to understand when to use each term in your writing.
Assume (Take for Granted Without Proof)
Most commonly, ‘assume’ means to take something for granted without evidence. For example, imagine you pushed Geoff into a swimming pool only to see he was struggling to stay afloat. After the lifeguard had dragged him to safety, you might say:
I assumed he could swim!
Here, ‘assume’ would be correct because you had no reason to think Geoff could swim.
A less common use of ‘assume’ is to mean ‘take on’ or ‘adopt’, particularly when it comes to a responsibility or characteristic:
After Susan was fired, David assumed the managerial role.
Though this usage is less common, it’s worth remembering in formal or academic contexts.
Presume (Suppose Based Upon Probability)
A ‘presumption’ is based upon evidence, so we need a reason for believing something when using ‘presume’. For example, if Geoff told you that he’d been training in the swimming pool all day, you might say:
I presume you’re pretty tired, then?
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In this context, the fact Geoff has been swimming all day lets us make an informed guess that he’s tired.
A less common use of ‘presume’ is to mean ‘act with unwarranted boldness’. For example, if you mocked Geoff’s haircut, he might respond by saying:
How dare you presume to criticise my mullet!?
This is a rather formal use of ‘presume’, but it’s worth knowing if you want to make a good impression in posh company.
Assume or Presume?
Choosing the right word here depends on how much prior information you have. If you are making a guess with no evidence, use ‘assume’. However, if you have a solid reason for guessing something, use ‘presume’. Remember:
Assume = Take for granted without proof
Presume = Suppose based upon probability