 As part of his attempt to understand the nature of possible worlds, Lewis (1986a, 81–86) categorizes different ways by which one can draw the abstract/concrete  distinction.[4]  These include: the way of example (which is simply to list the paradigm cases of abstract and concrete objects in the hope that the sense of the distinction will somehow emerge); the way of conflation (i.e., identifying abstract and concrete objects with some already-known distinction); the way of negation (i.e., saying what abstract objects are by saying what they are not, e.g., non-spatiotemporal, non-causal, etc.); and the way of abstraction (i.e., saying that abstract objects are conceptualized by a process of considering some known objects and omitting certain distinguishing features). He gives a detailed examination of the different proposals that typify these ways and then attempts to show that none of them quite succeeds in classifying the paradigms in accord with prevailing usage. Given the problems he encountered when analyzing the various ways, Lewis became pessimistic about our ability to draw the distinction cleanly.