What is the difference between real and nominal definitions?
Is it philosophically accurate to say that real definitions (as opposed to nominal definitions) can be true or false?
Take for example, the term "tiger." Assume I fix the denotation of the term and point to several large, four-legged cats. I then give the following real definition of "tiger": an eight-legged invertebrate. Is it correct to say that my provided real definition of "tiger" is false, assuming the prior denotation of tiger?
Your question reminds me of an old joke: if the tail of a tiger were defined to be a leg, how many legs would a tiger have? Answer: four, because defining a tail to be a leg doesn't make it one.
The point is that definitions are not true or false, just appropriate or inappropriate. Even a nominal definition is not arbitrary. The purpose of a good definition is to make a distinction where there is a difference and avoid making a distinction where there is not. Good definitions carve reality at the joints. So, defining a tiger to be an eight-legged invertebrate is entirely unhelpful, as is defining a tail to be leg, but not actually false. We might call such a definition idiosyncratic or bizarre, but if someone wishes to use such a definition for their own purposes, then that is their affair.
Of course, if one is talking about documenting the definitions of words as they are employed in common use, i.e. the sort of thing a lexicographer does when compiling a dictionary, then definitions can certainly be incorrect. In the normal sense in which "tiger" is used by English speakers, it definitely does not mean an eight-legged invertebrate.
See Definitions :
John Locke distinguished, in his Essay, “real essence” from “nominal essence.” Nominal essence, according to Locke, is the “abstract Idea to which the Name is annexed (III.vi.2).” Thus, the nominal essence of the name ‘gold’, Locke said, “is that complex Idea the word Gold stands for, let it be, for instance, a Body yellow, of a certain weight, malleable, fusible, and fixed.” In contrast, the real essence of gold is “the constitution of the insensible parts of that Body, on which those Qualities [mentioned in the nominal essence] and all other Properties of Gold depend (III.vi.2).”
A rough way of marking the distinction between real and nominal definitions is to say, following Locke, that the former states real essence, while the latter states nominal essence. The chemist aims at real definition, whereas the lexicographer aims at nominal definition.
This characterization of the distinction is rough because a zoologist’s definition of “tiger” should count as a real definition, even though it may fail to provide “the constitution of the insensible parts” of the tiger. Moreover, an account of the meaning of a word should count as a nominal definition, even though it may not take the Lockean form of setting out “the abstract idea to which the name is annexed.”
Perhaps it is helpful to indicate the distinction between real and nominal definitions thus: to discover the real definition of a term X one needs to investigate the thing or things denoted by X; to discover the nominal definition, one needs to investigate the meaning and use of X.
Whether the search for an answer to the Socratic question “What is virtue?” is a search for real definition or one for nominal definition depends upon one’s conception of this particular philosophical activity. When we pursue the Socratic question, are we trying to gain a clearer view of our uses of the word ‘virtue’, or are we trying to give an account of an ideal that is to some extent independent of these uses? Under the former conception, we are aiming at a nominal definition; under the latter, at a real definition.
Your purported "eight-legged invertebrate" definition pf "tiger" is false due to zoologist’s (real) definition of “tiger” that is currently "agreed" by the scientific community as the correct one.
Given that nominal definitions describe something in terms of its properties instead of its underlying essence:
"An eight-legged invertebrate"
wouldn't be a false definition when defining the word "tiger", just a nominal one. Nominal definitions aren't false, they just aren't exact. It would, however, be false to call the former a "real" definition.
