The living occupy the world of nature, but the world of thought is also inhabited by the dead, and especially by the ‘mighty dead’, the founders and ancestors; indeed in this world they are no longer dead, they live on, they appear in dreams and waking dreams, they are immortal – and shall we too not be immortal, in that other world? The supernatural appears, with its gods and devils, heavens and hells, in that second world of thought and imagination. In memory the past, that which has vanished from the perceived world, continues to exist – where does it exist? In that other world which we perceive only in thought. The ties and limitations of the physical world disappear in that other world, we are free of them, we can do anything; but at the same time it is only there that we encounter actual terror, the terror which hovers about the abode of the god and which stalks the sleeper when, dead to the physical world, he enters that other world wholly and wanders about in it. Is it any wonder, then, that this second world, the world of thought, comes to be much more real than the physical world, that men should come to regard it as alone real, as ‘the real world’? And once this step has been taken, is it not likely that the physical world will be progressively devalued for the benefit of the ‘real’ world, that all the qualities in it which men find useful or interesting or dreadful will be transferred one by one to the ‘real’ world as to their true home, so that at last the physical world is denuded of all value and claim to veneration and becomes a mere illusion, an appearance, a veil masking that other, ‘real’ world?

Before philosophy, before rational thinking at all, it is there: the world as appearance and the world as reality. Mythology and primitive religion speak directly of this transference of value; philosophy and sophisticated religion do so more covertly. By the sixth century B.C. the idea of two worlds is so firmly a part of human consciousness that even Kung Fu-tse, the very model of the secular prophet, assumes it, and the Buddha in India, Zarathustra in Persia and Isaiah in Palestine preach it constantly. In Ionia the content of the idea undergoes a remarkable change, but the form remains the same: the physiologoi, as Aristotle names them, the ‘investigators of nature’, begin to speak the language of science and philosophy, but still they answer the old metaphysical question ‘Is the world what it appears to be?’ in the negative. ‘Reality’ is still something that lurks behind the scenes. Thales says that reality is water. Heraclitus says it is ‘an ever-living fire’ and the ‘real world’ the battleground for an eternal conflict and for ceaseless change: change is ‘real’; unity and permanence are ‘appearance’. According to Parmenides change is logically impossible, an illusion: ‘reality’ is motionless, unchanging being. Anaxagoras taught that all four ‘elements’ are present in everything, so that water also contains earth, air and fire, though in small quantity compared with the quantity of water: water appears to be water; in reality it is a compound of earth, air, water and fire. Democritus propounded the atomic theory, or the doctrine that solidity is appearance. Only this tradition – the tradition that the seeming is not the real – can, with an ingredient from the old-fashioned mysticism of Pythagoras and Empedocles, account for the philosophy of Plato, in whose doctrine of supra-sensible Forms the primitive notion of a real and an apparent world receives its most coherent expression. Everyone knows Plato's parable of the men sitting in a dark cave with their backs to the light watching the play of their shadows: the shadows are the ‘world’, the light comes from the ‘real world’. In Plato the physical universe has been devalued to a shadow-play on the wall.

Platonism is the conceptual basis of Christianity. The actual bridge is the so-called Neo-Platonism of Plotinus, in which the crumbling Roman Empire is contrasted with the supernal realm of beauty and order which alone is asserted to be real. Plotinus's real world is almost the Christian heaven, his Roman Empire almost the Christian kingdom of this world: all that is lacking is a god. With the victory of Christianity all positive value is transferred to the Beyond. Moreover: ‘Now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known’ – in other words, this world is a veil obscuring the light of reality. From Paul to Augustine the history of Christianity is the history of a progressively widening gap between the two worlds, until in its final form Christianity means duality: the duality of Church and State, Pope and Emperor, this world and the next, the City of God and the Earthly City, God and his creation, the individual souls of men and the Holy Spirit ‘in whom we are all one’ – all ultimately forms of that primeval duality of the world in the eyes and the world in the mind.

Modern philosophy begins by restoring to this primeval duality its simplest form of expression: Descartes’ solution to the riddle of the universe is that there are two and only two existants in the world of created things: thinking substance or soul and extended substance or matter. These are the two worlds of man undisguised. Descartes is very sober – a great virtue after the metaphysical debauch of the Middle Ages. He does not even suggest that the thinking substance is ‘more real’ than the extended substance: appearance and reality is not a problem for him, or rather it is a problem he refuses to get involved in. How are the thinking and the extended substances, soul and matter, united in man? Descartes says he does not know and turns to other things. But ‘I don't know’ is water, and mankind wants something stronger: ‘I don't know’ won't do. Spinoza, the subtlest of all distillers of metaphysics, provides what is needed: soul and matter, says Spinoza, are not substances; there is only one substance, God; God is infinite and has an infinite number of modes of being; of these we know two, thought and extension; soul and matter are therefore modes of one underlying substance. Everything is ‘really’ God.

The relationship between Descartes and Spinoza is repeated in the relationship between Locke and Berkeley, and it is through this line that the great problem ultimately reaches the ears of Schopenhauer. Locke starts from the material world, which to him is what is given and self-evident.