That is "inside" which belongs to the subjective order, and is contrasted with the former.
If we deny that there is an objective order, an external world, and say that everything is "inside," we lose our distinction, and even the word "inside" becomes meaningless.

It indicates no contrast.

When men fall into the error of talking in this way, what they do is to _keep_ the external world and gain the distinction, and at the same time to _deny_ the existence of the world which has furnished it.

In other words, they put the clerk into a telephone exchange, and then tell us that the exchange does not really exist.

He is inside--of what?
He is inside of nothing.