People rarely say every step out loud. In a chain of reasoning or a short argument, a premise is often left for the listener to fill in.
Key idea
An unstated premise is a claim the argument needs but does not say. Supply the most reasonable one: it should connect the stated premise to the conclusion, and the speaker should accept it.
Why it works
Without the missing link, the stated premise does not reach the conclusion. "Rex is a dog, so Rex is a mammal" works only because we all know that dogs are mammals. Writing it down lets us check it, which is the whole point.
Worked example: Fill the gap
"Mia is a member of the band, so Mia must go to Friday practice."
- Stated premise: Mia is a band member. Conclusion: Mia must go to Friday practice.
- What links them? A claim about band members and Friday practice.
- Most reasonable: "Every band member must go to Friday practice."
Answer: Unstated premise: "Every band member must go to Friday practice."
Worked example: Not too strong
"The milk smells sour, so we shouldn't drink it." Which missing premise is best: "Sour-smelling milk is not safe to drink" or "We should never drink milk of any kind"?
- Both would make the conclusion follow.
- The second is far stronger than the speaker needs, and the speaker clearly does not believe it.
- Charity and accuracy both pick the first.
Answer: "Sour-smelling milk is not safe to drink."
Common mistake
Supplying a premise that just repeats the conclusion ("We shouldn't drink the milk"), or one much stronger than the speaker would accept. Check: does it link the stated premise to the conclusion, and would the speaker say "Yes, that's what I meant"?