Epistemology is concerned with questions about what can we know and how can we know it. One school of thought, rationalism, postulates an active mind, one that organizes sensory input in such a way as to add new meaning beyond the sensory data itself. The mind is said to contain some sort of innate cognitive capacity that is a precondition for knowing. A reasoning, active mind is primary, the true source of knowledge. Sensory experience in and of itself is deemed unreliable. The active mind is contrasted with the passive mind postulated by radical empiricists, that reflects upon sensory input but adds nothing new to the experience. This dichotomy in Western thought began with René Descartes (1596-1650) and John Locke (1632-1704), and has continued as a debate between a priori (rationalist and nativist) and a posteriori (empiricist) origins of knowledge, respectively.