In his Companion to a Higher English Grammar, a most delightful and erudite read by the way, Alexander Bain considers whether to define a noun by its meaning or its office, that is, by what objects a noun names or by what function a noun plays in a sentence.
He first does admirable work unfolding the oddity of the common stock definition (person, place, or thing), walking through several variations of the definition and then suggesting categorical ambiguity and duplication resulting from the term ‘thing’.
After this, and in part because of the difficulties of creating a list of objects for a noun, he declares that the true grammatical definition of noun is its role as possible subject (or) object in a sentence.
Thus a noun receives a functional definition. I have no great bone to pick and find this very helpful, however, other problems arise.
First, we can note that a noun is no unrecognizable accept within an actual sentence. We cannot determine a noun, but by the role it plays (or more generously potentially plays). But gone are the days that we point to a name and say, that is a noun.
Second, he rightly says we must be careful to distinguish noun from subjective infinitives as in the sentence, “To run is an excellent exercise.” But how do we go about doing this, I ask?
If we have only the functional definition remaining to us, we can no longer say, “no, that is a verb being used as a subject.” We must say, that is a noun. because it is a subject.
Further, I ask, how did we know it was the subject and how recognize the verb (‘is’)?
We are left with two dangers, on the one hand ambiguous definitions which only give a list of possible objects of representation by nouns, and actions, states, etc., by verbs, or we define by office performed in a sentence, but then give no explanation how we recognize they perform such an office.
While the second is more truly grammatical, it is a logical grammar, devoid of necessary connections between words and things and between living language and speakers or grammarians in this case.
Oh, poor lost Noun, I hardly know thee anymore!
Same & Other 