Thus a descriptive phrase is part of the proposition which it helps to express, whereas a demonstrative phrase is not part of the proposition which it helps to express.
Again the expositor might be standing in Green Park--where there are no college buildings--and say, 'This college building is commodious.' Probably no proposition will be received by the recipient because the demonstrative phrase, 'This college building' has failed to demonstrate owing to the absence of the background of sense-awareness which it presupposes.
But if the expositor had said, 'A college building in Green Park is commodious,' the recipient would have received a proposition, but a false one.
Language is usually ambiguous and it is rash to make general assertions as to its meanings.

But phrases which commence with 'this' or 'that' are usually demonstrative, whereas phrases which commence with 'the' or 'a' are often descriptive.

In studying the theory of propositional expression it is important to remember the wide difference between the analogous modest words 'this' and 'that' on the one hand and 'a' and 'the' on the other hand.

The sentence 'The college building in Regent's Park is commodious' means, according to the analysis first made by Bertrand Russell, the proposition, 'There is an entity which (i) is a college building in Regent's Park and (ii) is commodious and (iii) is such that any college building in Regent's Park is identical with it.' The descriptive character of the phrase 'The college building in Regent's Park' is thus evident.

Also the proposition is denied by the denial of any one of its three component clauses or by the denial of any combination of the component clauses.