Halliday & Matthiessen (1999: 263):
In the semantic construction of experience, ‘process’ and ‘participant’ emerge as prototypical categories […]. But as in any semiotic endeavour there are always some domains of uncertainty; are rain, wind, thunder processes or things? Are fear, worry, regret processes or qualities? Examples like these prevent the categories from being too reified and rigid, and provide a kind of gateway of analogy through which a phenomenon can drift or be propelled from one category to another.
In transcategorisation some other semantic feature triggers the propulsion; e.g. dark + make/become = darken, flake + like/composed of = flaky. In metaphor, however, the phenomenon is reconstrued as another category; what is being exploited is the potential that arises — but only after the categories have first been construed as distinct; not otherwise — of treating every phenomenon in more ways than one. In this process the original interpretation is not supplanted; it is combined with the new one into a more complex whole.