  Peirce maintained a considerable interest in the topic of classification or taxonomy in general, and he considered biology and geology the foremost sciences to have made progress in developing genuinely useful systems of classification for things. In his own theory of classification, he seemed to regard some sort of cluster analysis as holding the key to creating really useful classifications. He regularly strove to create a classification of all the sciences that would be as useful to logic as the taxonomies of the biologists and geologists were to these scientists.  Of special interest in this regard is the fact that he considered the relation of similarity to be a triadic relation, rather than a dyadic relation. Thus, for Peirce taxonomies and taxonomic trees are only one sort of classificatory system, albeit the most highly-developed one. He would not be in the least surprised to find that the topic of constructing “ontologies” is in vogue among computer scientists, and he would applaud endeavors to construct ontologies. He would not find in the least alien many contemporary analytic discussions of the notion of similarity; he would be right at home among them.