I know various contingent truths about my environment by perception. For example, by looking, I know that there is a computer before me; by hearing, I know that someone is talking in the corridor; by tasting, I know that the coffee has no sugar. I know these things because I have some built-in mechanisms specialized for detecting the state of my environment. One of these mechanisms, for instance, is presently transducing electromagnetic radiation (in a narrow band of wavelengths) coming from (...) the computer and the desk on which it sits. How that mechanism works is a complicated story—to put it mildly—and of course much remains unknown. But we can at least produce more-or- less plausible sketches of how the mechanism can start from retinal irradiation, and go on to deliver knowledge of my surroundings. Moreover, in the sort of world we inhabit, specialized detection mechanisms that are causally affected by the things they detect have no serious competition—seeing the computer by seeing an idea of the computer in the divine mind, for example, is not a feasible alternative. (shrink)