Presume vs assume: the difference in meaning, with examples

Presume and assume both mean to take something as true without proof; the difference is what you imply about your grounds. Say a colleague's desk is empty at ten. I presume she's at the dentist tells the listener you have a reason, such as her calendar or something she said yesterday. I assume she's at the dentist admits you're filling the gap without one, or setting it down as a working premise.

Merriam-Webster's usage note draws the same line: presume is for an informed guess based on reasonable evidence, assume for a guess based on little or no evidence. The line isn't fixed. In everyday speech the two overlap, and nobody will stop you for saying “I presume it'll rain.” The distinction matters most in writing someone may act on, such as a report, a contract or a news story, where the reader needs to know whether you had evidence. A quick test: if “from what I can see” fits, write presume; if “for now” or “for the sake of argument” fits, write assume.

Each word also has a second job the other can't do. Presume can mean to dare or to overstep (I wouldn't presume to tell you how to run your kitchen), which is where presumptuous comes from. Assume can mean to take on or to put on: assume responsibility, assume office, an assumed name.

Presume vs assume at a glance

Presume: a guess with reasons

In the supposing sense, presume says you are reasoning from what you know. Merriam-Webster's definition is about expecting something with confidence.

• The lights are off, so I presume they've gone to bed.
• The parcel was signed for, so we presume it arrived.
• Nobody has sent an agenda; the meeting is presumably off.

The adverb presumably carries the same confidence: probably, as far as anyone can tell. It is often the more natural choice in a report, where I presume can sound formal or arch.

Presume meaning “dare”: presumptuous and presumptive

The earliest meaning Merriam-Webster records, from the 14th century, is to undertake something without leave or clear justification: to dare. The word goes back to the Latin praesumere, from prae-, “before,” and sumere, “to take.” That is the presume in I won't presume to advise you, and in presume on or presume upon, to take advantage of someone's goodwill: I don't want to presume on your hospitality for another week. Its adjective, presumptuous, describes someone who takes liberties: It would be presumptuous to book the venue before she says yes. An email that closes with “Thanks in advance” for a large favor can sound presumptuous for the same reason, as the guide on how to end an email points out.

Presumptive is a different adjective: based on a reasonable presumption. A presumptive nominee is the candidate expected to win the nomination before it is official. Mixing up presumptive and presumptuous is an easy slip, and an unflattering one for the candidate.

Presumed innocent: the legal meaning

Law uses presume for a starting point the court is bound to apply. Merriam-Webster's legal definition is “to suppose to be true without proof or before inquiry.” In a criminal trial, the presumption of innocence means the defendant starts out innocent in the eyes of the law, and the prosecution carries the burden of proving otherwise. That is why the phrase is presumed innocent, never “assumed innocent.” The US Supreme Court put the principle this way:

Assume has legal uses too, in its taking-on sense: a buyer can assume a mortgage, taking over the seller's loan, and a person who joins a risky activity knowingly may be said to assume the risk. Neither involves supposing anything.

Assume: a premise or a role

In the supposing sense, assume is the weaker word. Merriam-Webster says it is used when the guess rests on little or no evidence:

• I assumed the café took cards. It didn't.
• Don't assume the reader knows the acronym.
• Everyone assumed the quiet one had written the memo.

That weakness is also its strength. In reasoning, an assumption is a premise you adopt on purpose and say out loud: Assume, for the sake of argument, that the figures are right. Mathematicians write assume n is even; financial models list their assumptions up front. A stated assumption is a starting point a reader can check. An unstated one is a gap, which is what Toulmin's “warrant” exposes in the argumentative essay examples.

The second sense: to take on. Assume comes from the Latin assumere, from ad- and sumere, “to take,” and the earliest meaning Merriam-Webster records, from the 15th century, is to take something up or receive it. Today the taking-on sense covers responsibility, roles and power (assume command, assume office, assume control of the company), a shape or a manner (the clouds assumed the outline of a ship, she assumed an air of calm) and a false identity (he traveled under an assumed name). The adjective unassuming means modest, someone who claims nothing for themselves.

Presume and assume in classic texts

Stanley had led an expedition inland specifically to find Livingstone, and they met at Ujiji, near Lake Tanganyika, in 1871. His guess came with evidence, which is exactly when presume fits. Whether he really said it is another matter. The line doesn't appear in his journal from the time, the two pages after his first sighting of Livingstone were torn out, and his biographer Tim Jeal believes he made it up afterward.

Read “suppose” into the first line and it makes no sense. Pope means “don't dare to examine God”: the older sense of presume, still alive in presumptuous.

Hamlet tells his mother to act the part of virtue until habit makes it real. Brontë's housekeeper simply puts on her best clothes, a use that sounds old-fashioned now.

Examples in sentences

Presume, supposing with reason:

1. Her coat is gone, so I presume she's left for the day.
2. The defendant is presumed innocent until the state proves otherwise.
3. The missing hikers are presumed to have sheltered in the old mine.
4. You're Ana's brother, I presume? You have her laugh.
5. The second draft, presumably the one with the new ending, is in the shared folder.

Presume, daring or overstepping:

6. I wouldn't presume to rewrite your vows for you.
7. He presumed on their friendship to borrow the car again.
8. It was presumptuous of her to announce the merger before the vote.

Assume, supposing with little or no evidence:

9. I assumed the store opened at nine; it opens at ten.
10. Never assume an editor has read the earlier chapters.
11. Most readers will assume the narrator is telling the truth.
12. Let's assume, for now, that the shipment arrives on time.
13. The forecast assumes inflation of 3 percent a year.
14. Assume the worst and pack a spare charger.

Assume, taking on or putting on:

15. She will assume the role of managing editor in March.
16. The deputy assumed command when the captain fell ill.
17. He wrote three crime novels under an assumed name.
18. The buyer agreed to assume the existing mortgage.
19. The fog assumed the shape of the harbor wall.
20. For an unassuming man, he had a lot of opinions about fonts.

Both in one sentence:

21. I presume you've read the report, but don't assume the board has.
22. We can't presume consent; we have to ask, and assume nothing.
23. Presumably the new director will assume her duties after the audit.

Mistakes to look for

• I wouldn't assume to tell you how to cook. → I wouldn't presume to tell you how to cook. (The “dare” sense belongs to presume.)
• He's the presumptuous nominee. → He's the presumptive nominee. (Presumptuous means overbold.)
• It seemed presumptious. → It seemed presumptuous. (The spelling has a u: presump-tu-ous.)
• Assumed innocent until proven guilty → Presumed innocent until proven guilty
• She assumed on his kindness. → She presumed on his kindness.
• The model presumes 2 percent growth. → The model assumes 2 percent growth. (A stated premise, not an inference.)

Quick quiz: presume or assume?

1. The new CEO will ____ her duties on Monday.
2. Her name is on the door, so I ____ this is her office.
3. Under the law, the accused is ____ innocent.
4. I don't want to ____ on your generosity.
5. Let's ____ the client says yes, and draft the schedule.
6. He wrote under an ____ name for twenty years.
7. It would be ____ to sign the letter on the board's behalf.
8. She ____ an air of calm she didn't feel.

Find the assumptions in a draft

The useful question for your own writing isn't only which word to choose. It's which of your claims rest on evidence and which rest on nothing yet. For each one in the second group, add the evidence, mark it as an assumption (we assume, this estimate assumes) or cut it. A decision record is a good place to keep the assumptions a project depends on.

In Strokes, you can ask the Strokes Assistant to review a section and list the statements that look unsupported, which gives you that second group to work through.

More pairs that are easy to swap: advice and advise, further and farther and principal and principle. If your publication has a rule on any of them, add it to your style sheet.