It is substituting a vis a tergo for the evolution of living force.
When a man, instead of burning up three hundred pounds of carbon a year, has got down to two hundred and fifty, it is plain enough he must economize force somewhere.

Now habit is a labor-saving invention which enables a man to get along with less fuel,--that is all; for fuel is force, you know, just as much in the page I am writing for you as in the locomotive or the legs that carry it to you.

Carbon is the same thing, whether you call it wood, or coal, or bread and cheese.

A reverend gentleman demurred to this statement,--as if, because combustion is asserted to be the sine qua non of thought, therefore thought is alleged to be a purely chemical process.

Facts of chemistry are one thing, I told him, and facts of consciousness another.