|Similarity: conformity of one or more - but not all - properties of two or more objects.|
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|Nozick II 660
Similarity/categories/Tverski: two things can be the most similar within a group while in another group they are the most dissimilar - therefore, the formulation of a category must not only be about the degrees of similarity, but must also cover of degrees of dissimilarity.
Difference: places things in different classifications.
Tverski: adding or removing some objects can affect how other things are classified - clusters are typically designed so that they maximize the similarity within the cluster and increase the difference to other clusters.
Philosophical Explanations Oxford 1981
The Nature of Rationality 1994