However it is clear to Locke that colour, temperature etc are merely a result of the interaction of particles (and their primary qualities) with our sense organs. Our ideas of secondary qualities do not resemble actual qualities of the objects as these qualities are not present in the object itself. Also, for example, an idea can not be red because it has no microstructure and it has no power to produce a sensation in us - it simply is a sensation in us. .
Locke makes a clear distinction between the two types of qualities and compiles a definitive list of each. Under primary qualities he lists, "solidity, extension, figure, motion/rest and number."" As secondary qualities he names "colour, sound, taste etc."" However, this list that was so immediately obvious to Locke is seen as problematic by many - particularly the list of primary qualities. For example, solidity is relative to temperature. This can be rectified by re-categorising solidity as impenetrability, i.e. the ability to keep any other thing out of the place where it is but the list is still far from satisfactory as many see motion as relative (objects appear to be moving faster when they are nearer to us). Also, many are confused by Locke's inclusion of "number- in the list as this is often considered to be a man made concept which we project onto the outside world. As a result of these difficulties, the main focus of this essay will be on whether a distinction can be made at all between primary and secondary qualities rather than on what, specifically, should be included under such headings. .
Bishop George Berkeley is one of the main opponents to Locke's theory and argues, on various grounds, that a distinction between primary and secondary qualities can not reasonably be made. He was motivated by his fear that wide-spread allegiance to such a scientific theory of the world could result in atheism and immorality.