  On this model, in seeking to define anything, we work back up the  appropriate classificatory hierarchy to find the higher (i.e., more  basic or more general) ‘Forms’, by means of which we can  lay down the definition. Although Plato did not himself use the term  ‘analysis’—the word for ‘division’ was  ‘dihairesis’—the finding of the  appropriate ‘Forms’ is essentially analysis. As an  elaboration of the Socratic search for definitions, we clearly have  in this the origins of conceptual analysis. There is little  disagreement that ‘Human beings are rational animals’ is  the kind of definition we are seeking, defining one concept, the  concept human being, in terms of other concepts, the  concepts rational and animal. But the construals  that have been offered of this have been more problematic.  Understanding a classificatory hierarchy extensionally, that is, in terms of the classes of things denoted, the classes higher up  are clearly the larger, ‘containing’ the classes lower  down as subclasses (e.g., the class of animals includes the class of  human beings as one of its subclasses). Intensionally,  however, the relationship of ‘containment’ has been seen  as holding in the opposite direction. If someone understands the  concept human being, at least in the strong sense of knowing its definition, then they must understand the concepts  animal and rational; and it has often then seemed  natural to talk of the concept human being as  ‘containing’ the concepts rational and  animal. Working back up the hierarchy in  ‘analysis’ (in the regressive sense) could then come to  be identified with ‘unpacking’ or  ‘decomposing’ a concept into its  ‘constituent’ concepts (‘analysis’ in the  decompositional sense). Of course, talking of  ‘decomposing’ a concept into its  ‘constituents’ is, strictly speaking, only a metaphor (as Quine was famously to remark in §1 of ‘Two Dogmas of  Empiricism’), but in the early modern period, this began to be  taken more literally.