Meaning and Mental Representation Chapter 3

Problems for the idea that representation is founded on similarity:
(1) Makes truly radical misrepresentation impossible, allows for misrepresentation only when the dissimilarity is relatively small.
(2) The problem of the brain as medium: Similarity theory seems incompatible with physicalism. If mental representations are physical things, and if representation is grounded in similarity, then there must be physical things in the brain that are similar to the things they represent. But this could only work if the mind-stuff is nonphysical. And "restricted" similarity (like pictures, cartoons) won't work because it is only "perceived" similarity.
(3) Similarity theories cannot deal with abstraction. How can a representation represent a whole class of things that differ widely from another on many dimensions? How do we rule out resemblance in irrelevant aspects?
For Locke, the problem of abstraction and the problem posed by secondary qualities lead to the covariance theory solution.