Seventeenth century empirical philosophers believed that secondary qualities are what we perceive—namely colors, sounds, textures, and smells. They thought that primary qualities like mass and number were the qualities of atoms that made up objects. We can’t perceive primary qualities, but the seventeenth century empiricists held that it is the interaction between the primary qualities of atoms that cause our perception of secondary qualities. For example, the atoms in red paint interact with our eyes, through light, to cause the experience of red. But Berkeley denied that there was a distinction between primary and secondary qualities because it is impossible to have an idea of a primary quality such as mass, extension, size, or number without also having an idea of its color, texture, or other secondary qualities.