The locution “nothing but” is frequently used when philosophers discuss appearances. The appearances are said to be “nothing but” particles or corpuscles, for example, or structured brain events. Even Thomas Hobbes, who recognized and honored the appearances, employed the “nothing but” locution frequently. That locution did not mean he denied the appearances or reduced them to matter and motion. Hobbes’s materialism is at best an explanatory one, not an ontological one. He was very firm: there are appearances (phantasms) and reality (matter and motion). Our contemporary materialists are not so clear about what they are affirming or denying. Often, they seem to me to confuse two claims: (a) all phenomena, all seemings or appearings, can be explained in terms of or by reference to, e.g., brain events, and (b) there are only brain events (and other physical events in the environment). The recent vogue for talking about supervenience may be an attempt to have it both ways, somehow to combine (a) and (b). Perhaps the appeals to supervenience are a genuine recognition that phenomena, qualia and mental events are also real, also exist.

To follow claim (a) rigidly may eliminate the need for any causal explanation of appearances, qualia or awareness. Whether supervenience is a causal relation, I am unclear. Most often, it seems to be treated as an explanatory relation: awareness or consciousness arises from, or emerges out of, a specific organization and structure of brain processes. But whatever the relation is, to talk of supervenience would seem to lead to the recognition that what supervenes, what arises from, differs in some ways from that from which it has emerged, or what it supervenes on: the supervenee and the supervened would seem to differ, at least numerically. With perceptual qualia or phenomenal properties, the difference cannot just be numerical. There is a kind difference between seen color or heard sound and the physical and neural events that precede our experience of color or sound. Similarly, being aware of tables, computers, or coffee differs in kind from the physical and neural processes that correlate with such awareness.

John W. Yolton, Realism and Appearances: An Essay in Ontology