Since the 17th century, a central issue in metaphysics and epistemology has concerned the status of what, following John Locke, have come to be called “secondary qualities”: colors, odors, tastes, sounds, warmth and coldness, etc. The problem is posed by the fact that while these qualities are experienced as belonging to objects in our external environment – the apple is experienced as red, the rose as fragrant, the lemon as sour, etc. – the scientific world-view, as developed in the 17th century and subsequently, seems to allow no place for them among the objective properties of material bodies. A common solution has been to deny, in one way or another, that these qualities do objectively inhere in external things in the way we experience them as doing. Galileo seems to have held that colors, etc., are properties of our sensations rather than of external things. Locke sometimes wrote as if that were his view as well, although his official view, as I read him, was that colors and the like are “powers” in objects to produce certain sorts of “ideas” in us.

On a natural understanding of it, the difference between Locke's view and Galileo's is semantical rather than metaphysical. Both hold that our sensations or ideas have what for the moment I will call subjective properties, which are immediately accessible to our consciousness and have no resemblance to any properties inhering in external objects, and both would allow, no doubt, that in virtue of the properties that do inhere in external objects, what Locke called their “primary qualities,” such objects have powers to produce in us sensations or ideas having certain of these subjective properties.