To see the full force of his reasonings one must read some work setting forth the history of the atomic theory.
If, then, we ask the question: What is the real external world?
it is clear that we cannot answer it satisfactorily without taking into consideration the somewhat shifting senses of the word "real." What is the real external world to the plain man?
It is the world of touch things, of objects upon which he can lay his hands.

What is the real external world to the man of science?
It is the world of atoms and molecules, of minuter touch things that he cannot actually touch, but which he conceives as though he could touch them.
It should be observed that the man of science has no right to deny the real world which is revealed in the experience of the plain man.

In all his dealings with the things which interest him in common life, he refers to this world just as the plain man does.

He sees a tree and walks towards it, and distinguishes between its real and its apparent color, its real and its apparent size.

He talks about seeing things as they are, or not seeing things as they are.