Since the first appearance of Locke’s Essay Concerning Human Understanding, his account of the idea of substance has probably generated more criticism and disagreement than any other single topic in that book. If this seems strange from a present-day philosophical perspective, it is only because in recent times the idea of substance has played a much less prominent role in metaphysics than it did throughout the seventeenth century. Its philosophical importance at that time was intimately linked with its central place in theological thought, where it was invoked in accounts of the nature of God and the immortality of the soul. Thus the doctrine of transubstantiation – according to which bread and wine can become the body and blood of Christ – and the doctrine of the Trinity – according to which Father, Son and Holy Ghost are three distinct persons in one indivisible Godhead – both depend for their intelligibility on the idea of substance and accordingly risk being undermined if that idea is challenged. The surest way to protect the idea from criticism, it might seem, would be to declare it innate. Consequently, to the religious dogmatists of Locke’s time, his attempt to provide the idea with a merely empirical foundation must have appeared extremely dangerous, inviting theological dissent and opening the door to atheism.