David asked:

What did John Locke mean by secondary qualities in his Essay Concerning Human Understanding?

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Secondary qualities, according to Locke, are the "powers" or dispositions the primaryqualities of an
object — for example, shape — have to produce in an observer certain ideas. However, these ideas,
unlike the ideas of primary qualities, resemble nothing in the object itself. Thus, for example, color
was a secondary quality because although the idea of color (say red) was caused by the (primary)
qualities of the object, there was nothing "in" the object that the idea of red resembled. On the other
hand, the primary quality of shape does produce in the observer an idea (say oblong) that does
resemble something "in" the object itself.

Roughly: secondary qualities are not "really" qualities of the object, although their ideas are produced
in the observer by qualities of the object, whereas primary qualities are "really" qualities of the object.
So, objects are not "really" red, but they are "really" oblong.

Kenneth Stern