This article argues that Brentano’s classification of mental phenomena is best understood against the background of the theories of natural classification held by Auguste Comte and John Stuart Mill. Section 1 offers a reconstruction of Brentano’s two-premise argument for his tripartite classification. Section 2 gives a brief overview of the reception and historical background of the classification project. Section 3 addresses the question as to why a classification of mental phenomena is needed at all and traces the answer back to (...) Mill’s view that psychological laws are class-specific. Section 4 and 5 connect the second premise of Brentano’s argument to Comte’s principle of comparative likeness and Mill’s insistance that class membership is determined by the possession of common characteristics. And section 6 briefly discusses the evidence Brentano provides for the first premise. (shrink)