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No instruction in semantics is required to grasp that (1) and (2) do
not mean the same thing.
(1) Hesperus = Hesperus.
(2) Hesperus = Phosphorus.
Of course, it is one thing to note that sentences such as Hesperus = Hesperus
and Hesperus = Phosphorus differ in what they assert--and quite another
to say why this is so.
I will venture an explanation of this from the standpoint of a referential
view of meaning. Such a view holds that Hesperus and Phosphorus
are names, and that the meaning of a name is what it stands for.
In coming to terms with what sets (1) and (2)--and such pairs of sentences
generally--apart, I will adopt a referential view of meaning, not only
as is sometimes done for proper names like Hesperus and Phosphorus,
but for other singular terms3
as well.
On the view of meaning I have in mind, Hesperus will mean Hesperus
and Phosphorus, Phosphorus. As a result, (1) will assert, as intuitively
and pre-theoretically it appears to, that Hesperus is identical with Hesperus;
and (2) will assert, as intuitively and pre-theoretically it appears to,
that Hesperus is identical with Phosphorus. What sets apart (1) and
(2) will thus reflect some difference between the identity of Hesperus
and Hesperus and the identity of Hesperus and Phosphorus--and so, ultimately,
some difference between Hesperus and Phosphorus, which--notwithstanding
the historic principle of the Indiscernibility of Identicals4--are,
as (2) suggests, one and the same entity.
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