There is as yet no theoretically sound and empirically proven procedure that is fully reliable for the design of thickeners. From a logical standpoint, a sound procedure is one correctly deduced from valid premises or assumptions. As noted in Fitch's earlier work(979), there is no reason to believe that existing mathematical models are based on uniformly valid assumptions. There is also much reason to believe that they are not. Therefore existing mathematical models or “theories” of thickening are analyzed for mathematical consistency, scientific credibility, and relevance to chemical engineering. Domains in which the models are useful approximations of reality and thus are relevant to chemical engineering practice are identified. Emendations or alternatives are suggested to bring the models more nearly into conformity with empirical results. Design procedures are suggested that, although involving assumptions that cannot be uniformly valid, would seem more reasonable than existing ones.