This practical impossibility of pure demonstration is a difficulty which arises in the communication of thought and in the retention of thought.

Namely, a proposition about a particular factor in nature can neither be expressed to others nor retained for repeated consideration without the aid of auxiliary complexes which are irrelevant to it.
I now pass to descriptive phrases.

The expositor says, 'A college in Regent's Park is commodious.' The recipient knows Regent's Park well.

The phrase 'A college in Regent's Park' is descriptive for him.

If its phraseology is not elliptical, which in ordinary life it certainly will be in some way or other, this proposition simply means, 'There is an entity which is a college building in Regent's Park and is commodious.' If the recipient rejoins, 'The lion-house in the Zoo is the only commodious building in Regent's Park,' he now contradicts the expositor, on the assumption that a lion-house in a Zoo is not a college building.
Thus whereas in the first dialogue the recipient merely quarrelled with the expositor without contradicting him, in this dialogue he contradicts him.