He gives a negative reason. If we stipulate that the two sentences have the same meaning, the thin theory “is wholly without interest. Substantive philosophical questions cannot be answered by framing stipulative definitions.” Correct, but this begs the question as to whether there is any substantive philosophical question. A thin theorist is likely to be a positive or a nominalist, who wants to show how apparently ‘metaphysical’ questions really arise from a misunderstanding of language, or from being misled by it.
He goes on to give a positive reason.
‘A white man exists’ says all that ‘Some man is white’ says, but it says more: it makes explicit that there are one or more existing items that are such that they are both human and white. The existence-sentence is richer in meaning than the some-sentence. It makes explicit that the item that is both human and white exists, is not nothing, is mind-independently real -- however you want to put it.Will this work? I’m not sure. For the thin theorist, ‘there are one or more existing items’ and ‘there are one or more items’ or equivalent in meaning, by stipulation. The realist has failed to communicate anything.
On the point that “It makes explicit that the item that is both human and white exists, is not nothing, is mind-independently real”. Well, so does the ‘some’ sentence’. ‘Some man is white’ makes it explicit that the item that is both human and white exists, and that it is not nothing, and is mind-independently real. How could it say any less. If ‘some buttercups are blue’ is true, then blue buttercups exist (in virtue of the meaning alone), and so blue buttercups are not nothing, otherwise ‘no buttercups are blue’ would be true. And blue buttercups are mind-independently real, for ‘some buttercups are blue’ does not merely say that people think there are blue buttercups, or that they are figments of some kind. Over to Phoenix.
*We neo-scholastics call these ‘categorial’ and ‘existential’ sentences respectively. The medievals made a similar distinction between the use of the verb ‘is’ as a second elements, as in ‘Socrates is’, and as a third element or copula, as in ‘Socrates is white’.