In a recent paper Britton attempted to distinguish between a phantasy that has achieved the status of a belief and one that has not, and between a belief and knowledge. The author argues that, in the light of the seventeenth-century controversy between Descartes and Spinoza, both of these distinctions are untenable. Descartes argued, as Britton does, and as Freud did, that phantasies or ideas are not accepted as beliefs until they are tested against reality. Furthermore, Britton maintains that for a belief to acquire the status of knowledge, it must be supported by incontrovertible evidence. Spinoza, on the other hand, proposed the seemingly preposterous notion that a comprehended proposition is automatically believed. Doubt may subsequently be engendered by disconfirming evidence. As it turns out, research in a number of domains suggests that Spinoza was correct and Descartes was wrong. This evidence and its clinical implications are discussed. As suggested by Bion, instilling doubt regarding deeply ingrained (Spinozan-formed) phantasies is a principal goal of treatment.