The most basic existents in the Categories are particular substances and particular attributes. Meaning in the De Interpretatione is a function of reference to existents. Definitions of the basic objects of a demonstrative science must express real natures, according to the Posterior Analytics. Equally as much as science, language depends upon the apprehension of universals. Linguistic and scientific definitions articulate universal concepts. There is an apparent tension between the role of universals in Aristotle's epistemology and semantics and the Categories doctrine that universals are secondary and dependent upon human classificatory schemes. This is one problem Aristotle faces in the Metaphysics.

Another problem facing Aristotle is how to explain the relation between the meanings of the terms of natural language and the meanings of terms in scientific discourse (often the same linguistic sign has both uses.) Science requires meanings that correspond precisely to reality. Natural language requires words (at least designative ones) that refer to real existents. There can be no doubt that Aristotle distinguishes between the two cases. The nominal definition of a natural language term may express a concept that has no exemplifications, for instance, ‘goatstag’, or it may embody an imperfect understanding of the real existent, for instance, ‘human.’ What, then, is the difference between a meaning expressed in a linguistic definition and a meaning expressed in a definition of essence? This concern motivates the discussion of definition and essence in Metaphysics VII.