These words refer to the tentative adoption of an idea or interpretation in the face of incomplete evidence or uncertainty. Suppose is the most informal and general of these. It can be used, especially in speech, to present a proposal or opinion in a tentative way, making it come as a suggestion rather than as a directive.

• We’d better get going, I suppose .

The word can indicate also any hazarding of opinion, however well- or ill- informed. Guess is more likely to suggest a completely arbitrary notion or a lack of information or authority. Guess can even suggest a bluff or a groping in the dark without any certain hope of success: quiz programmes in which the contestants guess at the answers. Suppose, by contrast, more often suggests a shrewd notion based on some evidence: able to suppose how upset you were just from the expression in your eyes. Also, the word can point to something adopted experimentally or to something entertained simply for the sake of argument: asking him to suppose what he would do if he were in such a situation. Although suppose , like guess , implies a lack of certainty, there must be some grounds for supposing something, however, scanty.

• You’re only supposing this on hearsay; you have no proof.

Imagine exists in a wider context, indicating either a creative act or a deliberate entertaining of something totally contrary to known fact. In the present context, the word relates more closely to guess, suggesting a paucity of evidence on which a supposition is based: imagining all the wild stories of the neighbours to be true. Imagine emphasizes the role of invention or fancy in influencing perception: He imagined that he had heard a scraping noise. Even so, imagine carries over from its other contexts vague connotations of sympathy, understanding or reassurance, as in its common use as a stock phrase in speech.

• You’ll enjoy the film, I imagine.

When related to the context of argument or reasoning that surrounds suppose, imagine indicates a more thoroughgoing fabrication: largely imagining the existence of such a lost civilization, rather than demonstrating its existence.

Assume and postulate relate most closely to suppose in suggesting a context of argument or reasoning. Assume is the less formal of the two, emphasizing a conclusion based on little or no valid evidence; as such, it can suggest a general context pertaining to psychological rather than to logical mental operations: assuming that life owed him a living; She assumed he wouldn’t want to know her after their quarrel. In argument, the word is even more clear than suppose about agreeing to adopt a tentative stance in order to test a proposition: assuming the report to be valid simply to see how that would affect our policies in the next decade. In the stricter context of logic and reasoning, both postulate and assume point to those things that must be accepted as given prior to the reasoning process: preferring that explanation which requires us to postulate (or assume ) the fewest preconditions at the outset. Postulate is exclusively restricted in reference to the setting up of a theory in order to test its merit; it is therefore more precise than either assume or suppose : scientists who are traditionally driven to postulate a series of explanations until they find one that will hold.

Conjecture and surmise are comparable to postulate in formality, but comparable to suppose and guess in generality. Conjecture relates closely to guess , stressing the incomplete or inadequate evidence serving as a basis for judgement. A shrewd conjecture may include elements of intuition, extrapolations based on experience, a good sense of probability, and plain luck. When used without qualification, conjecture often indicates mere guesswork unsubstantiated by any evidence and without much credibility: aimless conjectures about conspiracies to overthrow the government, conjecturing how long the enemy would take to zero in on his position. Surmise relates to the aspect of suppose that indicates a shrewd notion based on evidence. The word points to an ability to detect clues and to infer valid conclusions from them; its connotations suggest detachment and an ability to reason dispassionately: surmising from their looks that he had called on them at an inopportune time. Surmise conveys a greater degree of certainty, or at least better grounds for drawing a conclusion, than any of the other words here considered.