assumed that you have direct knowledge of all the material things that you seem to perceive about you. It has never occurred to you that there are intervening physical agencies that you ought to take into account.
First-Hand Knowledge
When you look up at the clock, you instinctively feel that there is nothing interposed between it and your mind that is conscious of it. You seem to feel that your mind reaches out and envelops it.
As a matter of fact, your sense impression of that bit of furniture must filter through a great number of intervening physical agencies before you can become conscious of it.
Direct perception of an outside reality is impossible.
Second-Hand Knowledge
Before you can become aware of any object there must first arise between it and your mind a chain of countless distinct physical events.
Modern science tells us that light is due to undulations or wave-like vibrations of the ether, sound to those of the air, etc. These vibrations are transmitted from one particle of ether or air to another, and so from the thing perceived to the body of man.
Think, then, what crisscross of air currents and confusion of ether vibrations, what myriad of physical events, must intervene between any distant object and your own body before sensations come and bring a consciousness of that object's existence!