Whatever you know or think you know, of the external world comes to you through some one of your five primary senses, sight, hearing, touch, taste and smell, or some one of the secondary senses, such as the muscular sense and the sense of heat and cold. The impressions you receive in this way may be true or they may be false. They may constitute absolute knowledge or they may be merely mistaken impressions. Yet, such as they are, they constitute all the information you have or can have concerning the world about you.