 Starting in the early twentieth century, the notion of “sense data” played a central role in discussions of knowledge and of the ontology of perceptual experience. In epistemology, sense data were offered as a basis for all other sensory knowledge. They were epistemically privileged: many authors held that one could not be wrong about one’s sense data. Concerning what sense data are (their ontology), there has been a change of consensus. The early advocates of sense data (explicitly so-called) mainly considered them to be mind-independent, that is, to exist apart from the mind, perhaps as a special kind of thing, neither mental nor physical. In the latter part of the twentieth century, sense data came to be viewed most often as mind-dependent, as mental objects or contents of which we are directly aware in perceptual experience. Sense data dominated discussion of perception in the first half of the twentieth century. The notion is now frequently used as a stalking horse in the philosophy of perception, as an example of a position that should be avoided.