 Sensory qualities pose a serious problem for materialist theories of the mind. For where, ontologically speaking, are they located? Suppose Bertie is experiencing a green after-image as a result of seeing a red flash bulb go off; the greenness of the after-image is the quale. Actual Russellian sense-data are immaterial individuals; so the materialist cannot admit that the greenness of the after-image is a property of an actual sense-datum. Nor is it plausible to suggest that the greenness is exemplified by anything physical in the brain (if there is some green physical thing in your brain, you are probably in big trouble). To sharpen the problem: 