 The initial positing of sense data arose within an extant discussion of sense perception and its objects. In Oxford and Cambridge of the late nineteenth century, it was common to discuss perception as part of the “problem of the external world”, an epistemological problem found in modern philosophy, from Descartes onward, concerning whether we know an external world beyond our own minds and, if so, how we know it. These discussions typically assumed a distinction between sensation and perception, similar in many ways to that later drawn between sense data and the perception of material objects. A standard description of how vision occurs would say that sensations of shape and color are first produced via the stimulation of the sensory nerves, and that from such sensations the perceiver forms perceptions of objects, perhaps through a process of judgment or through learned associative connections among sensations. Let us turn, then, to the history of theories of sensation and perception, and the rise of sense data.