   Locke thought that our ideas of “primary qualities”—shapes, sizes, motions, textures and physical construction—resemble the qualities which cause them. However, according to Locke, our ideas of a variety of other qualities—colors, sounds, tastes and smells—do not. Ideas of colors, sounds, tastes and smells, or “secondary qualities,” are caused by certain complex configurations of primary qualities that bear no resemblance to the ideas which they cause. Reid is deeply struck by Berkeley's attack on this distinction at Principles of Human Knowledge 1.9–15. He agrees with Berkeley that no mental state or object could possibly resemble anything that is not, itself, a mental state or object. In Berkeley's famous words, “An idea can be like nothing but an idea” (Principles of Human Knowledge 1.8). Mental states and objects have only mental properties, but only something that is, itself, a mental state or object can have a mental property. Hence, nothing can resemble—that is, share a property in common with—a mental state or object other than another mental state or object. Berkeley took this point to show that no non-mental cause of an idea could resemble it, whether the relevant idea were an idea of a shape or a color. Berkeley concludes that Locke's distinction between primary and secondary qualities is a grand mistake.