The most important figure of speech, in which one subject-matter (sometimes called the tenor) is referred to by a term or sentence (the vehicle) that does not literally describe it: the ship of state, the light of faith, etc. Philosophical problems include deciding how the border between literal and metaphorical meaning is to be drawn (Nietzsche, for example, thought that literal truth was merely dead or fossilized metaphor), understanding how we interpret metaphors with the speed and certainty which we often manage, and deciding whether metaphors can themselves be vehicles of understanding, or whether they should be regarded only as signposts to literal truths and falsities about the subject-matter.

Particular disputes in philosophy can also centre on the extent to which a phrase is metaphorical, as when philosophers talk about the foundations of knowledge, beliefs in the head, the goodness of God, or abstract objects. In a mixed metaphor, or catachresis, the combination of properties suggested becomes illogical or ridiculous, although even then interpretation need not fail, as when Hamlet contemplates taking up arms against a sea of troubles.