Let us consider for a moment the imprisoned clerk.

He is in a telephone exchange, about him are wires and subscribers.

He gets only sounds and must build up his whole universe of things out of sounds.

Now we are supposing him to be in a telephone exchange, to be receiving messages, to be building up a world out of these messages.

Do we for a moment think of him as building up, out of the messages which came along the wires, those identical wires which carried the messages and the subscribers which sent them?
Never! we distinguish between the exchange, with its wires and subscribers, and the messages received and worked up into a world.