If what _looks like_ a beefsteak could _really be_ a fork or a mountain or a kitten indifferently,--but I must not even finish the sentence, for the words "look like" and "could really be" lose all significance when we loosen the bond between appearances and the realities to which they are properly referred.
Each appearance, then, must be referred to some particular real thing and not to any other.

This is true of the appearances which we recognize as such in common life, and it is equally true of the appearances recognized as such in science.

The pen which I feel between my fingers I may regard as appearance and refer to a swarm of moving atoms.

But it would be silly for me to refer it to atoms "in general." The reality to which I refer the appearance in question is a particular group of atoms existing at a particular point in space.

The chemist never supposes that the atoms within the walls of his test-tube are identical with those in the vial on the shelf.