ABSTRACT
The doctrine of sensations that would become the bedrock of H. von Helmholtz’s perceptual theory is a hybrid of these distinct, yet conceptually linked, lines of inquiry. Helmholtz interpreted the doctrine of specific nerve energies as a way to return to I. Kant’s metaphysics, but this time through physiology and psychology rather than mathematics and mathematical physics. The sensations of the senses are tokens for consciousness, it being left to our intelligence to learn how to comprehend their meaning. For Rene Descartes, sensation is a physical–mental hybrid. The development of the technical notion of sensation in the 19th century and the role ascribed to it within the theory of how we know the world by sight reflect this peculiar status. In the general case, unconscious inferences produce spatial precepts from non-spatial sensations. There are no discoverable connections, Thomas Reid argued, between sensations and the properties of objects that would justify the use of inference.
