A subject that is essential1 for the comprehension of the questions that we have mentioned, and of others of which we are about to speak, so that the essence of the problems may be understood, is this: that human knowledge is of two kinds. One is the knowledge of things perceptible to the senses—that is to say, things which the eye, or ear, or smell, or taste, or touch can perceive, which are called objective or sensible. So the sun, because it can be seen, is said to be objective; and in the same way sounds are sensible because the ear hears them; perfumes are sensible because they can be inhaled and the sense of smell perceives them; foods are sensible because the palate perceives their sweetness, sourness or saltness; heat and cold are sensible because the feelings perceive them. These are said to be sensible realities.