The first thing we must do then is to show that the human body is similar to the corporeal world. This will require an analysis of the structure of the latter. But before examining the _objects_ of knowledge, we must say a word about the process of knowing. Man perceives things in two ways--through sense and through intellect. His senses give him the accidents of things, the shell or husk, so to speak. He perceives color through sight, sound through hearing, odor through smell, and so on. It takes reason to penetrate to the essence of an object. Take as an example a book. The sense of sight perceives its color, and through the color its form. This is then apprehended by the power of imagination or representation. The latter in turn hands it over to the cogitative power of the rational soul, from the reflection of which results the spiritual reality of the object, which is its knowledge. So we see that the reason knows the essence and reality of a thing, whereas the senses know only its husk and its accidents. This same thing is stated by the philosopher in another form. The senses, he says, know only the particular, the universal can be known by the intellect only. This is because the soul is fine and penetrating, while the body is gross, and can reach the surface only. We may also classify knowledge from another point of view as necessary (or immediate), and demonstrated (or mediate). Necessary knowledge is that which no sane man can deny. Such knowledge may be of the senses, as the sight of the sun or the sound of thunder; or it may be of the reason, such as that the whole is greater than its parts. We may then enumerate four kinds of things known directly without the help of other knowledge, (1) The percepts of the senses. (2) Truths generally admitted by reason of their self-evidence. (3) Traditional truths, _i. e._, truths handed down by a reliable and wise man, or by a community worthy of credence. (4) First principles or axioms. These four can be easily reduced to two; for traditional truths ultimately go back to the testimony of the senses; while first principles or axioms are included in self-evident propositions. We thus have two kinds of necessary or immediate knowledge, the data of sense, and self-evident propositions. The latter kind is superior to the former, because man shares sense knowledge with the lower animals; whereas rational propositions are peculiar to him alone. Demonstrated knowledge is built upon necessary knowledge, and is derived from it by means of logical inference.[169]