Edward Ockham uses ‘Direct Reference’ to refer to "the theory that part or all of the meaning of a proper name requires the existence of a named object." This implies that a proper name cannot have a meaning unless there exists an object it names. He then gives the following argument:
A term signifies either a property or an object. But properties are repeatable. A property like being white, or running, or being bald can be instantiated by many individuals. Even a property that can only be had by one individual at a time (being the tallest living philosopher) can be instantiated by different individuals at successive times, or could be instantiated by a different individual than the one that possesses it now. If a proper name like 'Socrates' signified a property, even a unique property, it would make sense to say that this individual is Socrates on Tuesday, but that someone else is Socrates on Wednesday. Or that this individual is Socrates today, but might not have been Socrates. But that makes no sense. A proper name does [NOT] signify something that is repeatable, therefore does not signify a property. Therefore it signifies an object. Therefore an object is part or all of the meaning of a proper name, and the theory of Direct Reference, as defined above, is true.
As it stands, this argument is not compelling. To be compelling, it would have to close off the 'haecceity escape route.' Haecceitas is Latin for 'thisness.' Let us say that H is an haecceity property, an haecceity for short, if and only if H is a first-level property which, if instantiated, is instantiated by the same individual ('object' in Edward's terminology) at every time and in every possible world in which it is instantiated. Accordingly, 'the tallest living philosopher' does not express an haecceity property: it has different instances at different times and at different possible worlds, even though at a given time in a given world it has only one instance. If there are haecceity properties, then they are not repeatable, i.e., multiply instantiable, whether at different times or in different worlds.
Consider the property of being identical to Socrates. If there is such a property, it can serve as the sense of 'Socrates,' or, to use Edward's word, that which 'Socrates' "signifies." In the case of a vacuous proper name such as 'Vulcan,' the property of being identical to Vulcan could serve as its sense. If this is tenable, then 'Vulcan' is a genuine proper name despite it having no referent, and the Direct Reference theory as defined above is false.
Haecceities can either be nonqualitattive or qualitative. Identity-with-Socrates is an example of a nonqualitative haecceity. But one can imagine an haecceity property that is compounded out of qualitative properties where the latter are not tied to specific individuals in the way in which identity-with-Socrates is tied to the individual Socrates. The logical construction goes like this. We first form the huge conjunction K1 of all the qualitative properties that Socrates instantiates in the actual world. K has as conjuncts being snubnosed, being married, being a plebeian, being poor, etc. We do the same for every possible world in which Socrates exists. This yields a series of conjunctive properties, K1, K2, K3, etc. We then make a monstrous disjunctive property each disjunct of which is one of the Ks. This property is Socrates' qualitative haecceity. It is a property but it is clearly not repeatable (multiply instantiable). If there are such properties, they defeat Edward's argument above.
I myself do not believe in haecceity properties, nonqualitative or qualitative. See A Difficulty With Haecceity Properties. My point is that Edward's argument above is not compelling unless he can persuasively exclude them.
Now, given that I reject haecceity properties, I ought to find the above argument compelling. But this lands me in a quandry. For I hesitate to say that 'Vulcan' or 'Pegasus' are not proper names. They seem to be perfectly good proper names albeit vacuous. If so, then no part of their meaning involves the existence of a referent, and the DR theory is false.
Or consider 'Moses.' Was there some one man who received, or claimed to receive, the Torah from YHWH on Mount Sinai? Aren't we strongly tempted to say that the meaning of 'Moses' is what it is whether or not Moses existed? If we say that, then it can be no part of the name's meaning that it have an existing referent. Nor can it be any part of the name's meaning that there be a causal chain leading back to an initial baptism. If Moses never existed, then there was nothing to baptize.