Here goes, and bear with me if at first I seem irrelevant.
"Let us, first of all, investigate a particular industrial process, and whenever I state something with which you disagree, please interrupt me.

Here is a shoe factory.

This factory takes leather and makes it into shoes.

Here is one hundred dollars' worth of leather.

It goes through the factory and comes out in the form of shoes, worth, let us say, two hundred dollars.