Is this another one of John Locke's bad ideas (like the human mind as a "sheet of paper") or am I missing something here? http://en.wikipedia.org/wiki/Primary/secondary_quality_distinction That entry is brief and explains it well: Primary qualities exist and are independent of any observor, such as the shape of a table. Secondary qualities exist and are produced by the sensation of the observor, such as the color of the table. So, primary qualities are in bodies, the secondary qualities are in the percipient. So, without eyes there are no colors, without ears there is no sound, and so on. But, don't things such as sound, light, color and so on have properties that are measurable? For instance, our perception of color depends on the arrangement of the electrons and protons of the object and so on, not just psychological factors. You can even tell some properties of things such as fire by its color. I guess this wouldn't apply to people who are color blind, which might support Locke's position, but there is a chance he could utilize science to help "understand" color. There are some quotes on the wiki page saying that these properties wouldn't exist if living beings didn't exist to observe them. I believe the same case could be made that nothing exists if we didn't observe them. And, as Berkeley points out, as noted in the classic "History of Western Philosophy," many of these same properties exist to primary qualities, meaning they depend upon the observer as well. No two people would see the "shape" of the table exactly the same way, etc.