Realism has acquired a degraded and idealism an elevated meaning in common usage. The modern realist prides himself on his unblinking recognition of certain raw facts about man and nature: his self-appointed territory is the surface phenomena of things and he dare not, will not, travel beyond it. The idealist, by contrast, is an altogether freer spirit: regarding the elements of the real as though they were marionettes in his hands, he manipulates them at his will.

David Levy studiously avoids the mental trap of idealism while positing an Aristotelian realism which, unhappily, has been out of fashion these past three hundred years; ever since, in fact, Descartes made the clarity and distinctness of an idea the test of its correspondence with reality, thereby eliminating from the objective world everything which could not be described in the numerical language of 'res extensa', and hence every cause of Aristotle's except the 'efficient'. Hegel later took Descartes's mental leap a historical stage further and, as Levy puns, introduced us to the 'geist in the machine': an ego concerned less to mathematicize than to spiritualise the world.

Modern philosophy may be read as the story of how two contending schools of thought, the one rationalist and idealist, the other empiricist and positivist, have fed each other in a ceaseless dialectic. Even though Locke and, after him, Hume turned back to the experienced world from which the Cartesian ego had recoiled in suspicion, their more or less severe limitations ...