The Commens Dictionary
Quote from ‘Logic (exact)’
Copula is often defined as that which expresses the relation between the subject-term and the predicate-term of a proposition. But this is not sufficiently accurate for the purposes of exact logic. Passing over the objection that it applies only to categorical propositions, as if conditional and copulative propositions had no copula, contrary to logical tradition, it may be admitted that a copula often does fulfill the function mentioned; but it is only an accidental one, and its essential function is quite different. Thus, the proposition, “Some favoured patriarch is translated” is essentially the same as “A translated favoured patriarch is”; and “Every mother is a lover of that of which she is a mother” is the same as “A mother of something not loved by her is not.” In the second and fourth forms, the copula connects no terms; but if it is dropped, we have a mere term instead of a proposition. Thus the essential office of the copula is to express a relation of a general term or terms to the universe. The universe must be well known and mutually known to be known and agreed to exist, in some sense, between speaker and hearer, between the mind as appealing to its own further consideration and the mind as so appealed to, or there can be no communication, or “common ground,” at all. The universe is, thus, not a mere concept, but is the most real of experiences.