PurposeListener's needFunction of similePossibilitySuspects impossibilityShow an accepted analogous caseConditionDoes not know the kind of qualityIdentify its characterDegreeDoes not know its intensityClarify its amount or forceReinforcementKnows the condition, but weakly feels itMake the judgment more vivid
The comparison term is the clearer reference point in each case, but the questions differ: Is this possible?, What sort of state is this?, How much of it is there? and How can the established fact be made more persuasive? One familiar image does not automatically determine the rhetorical aim.