All advances of scientific understanding, at every level, begin with a speculative adventure, an imaginative preconception of what might be true — a preconception that always, and necessarily, goes a little way (sometimes a long way) beyond anything which we have logical or factual authority to believe in. It is the invention of a possible world, or of a tiny fraction of that world. The conjecture is then exposed to criticism to find out whether or not that imagined world is anything like the real one. Scientific reasoning is therefore at all levels an interaction between two episodes of thought — a dialogue between two voices, the one imaginative and the other critical; a dialogue, as I have put it, between the possible and the actual, between proposal and disposal, conjecture and criticism, between what might be true and what is in fact the case.

In this conception of the scientific process, imagination and criticism are integrally combined. Imagination without criticism may burst out into a comic profusion of grandiose and silly notions. Critical reasoning, considered alone, is barren. The Romantics believed that poetry, poiesis, the creative exploit, was the very opposite of analytic reasoning, something lying far above the common transactions of reason with reality. And so they missed one of the very greatest of all discoveries, of the synergism between imagination and reasoning, between the inventive and critical faculties.

Peter Medawar — Pluto’s Republic, “Science and Literature”, p. 46