An assumption is information you add without it being established by the problem. Some assumptions are natural in everyday conversation, but a must-follow reasoning question requires closer control over what has actually been given.
A familiar story can hide the gap
The library is open only when a librarian is present. A librarian is present. Must the library be open?
No. The first sentence makes a librarian's presence necessary for opening, not sufficient. The library could be closed for maintenance while a librarian works inside. The extra expectation that presence means opening came from outside the rule.
Watch for three common additions
- Reversal: treating if A then B as if B then A.
- Completeness: assuming the listed possibilities are the only possibilities when the question does not say so.
- Typicality: treating what usually happens as what must happen.
These additions can be subtle. A statement that several applicants passed does not tell you that everyone else failed unless the result list is explicitly complete.
Rephrase the information neutrally
Replace familiar nouns with letters or invented labels. If every courier has a badge becomes every A is B, it may become easier to see that having a badge does not prove someone is a courier.
This technique removes associations that can make an invalid inference feel natural.
Ask what would change your answer
If you need one extra sentence to make the conclusion certain, identify it. In the library example, a rule saying the library opens whenever a librarian is present would supply the missing direction.
Naming the missing premise is often more useful than simply calling an answer wrong.
Apply the distinction beyond puzzles
In ordinary decisions, assumptions may be reasonable working hypotheses, but they should remain visible. In a formal reasoning question, use the stated premises and conventions. The skill is not to avoid all assumptions forever; it is to know when your conclusion depends on one that has not been supplied.