Demonstrated knowledge is built upon necessary knowledge, and is derived from it by means of logical inference.[169]
We may now proceed to discuss the principles of the corporeal world. Matter is the foundation and principle of a thing. All things, natural as well as artificial, are composed of matter and form. Wood is the common matter of chair and bed. Their forms are different. So the common matter of the four elements is the prime matter endowed with the form of corporeality, _i. e._, with the capacity of filling place. This form of corporeality makes the prime matter corporeal substance. Matter is relative to form, form is relative to matter.
Spiritual things also have matter and form. In corporeal artificial things like ring or bracelet, the matter is gold, the form is the form of ring or bracelet, the efficient cause is the art of the goldsmith, the final cause or purpose is the adornment. In spiritual things we may compare genus to matter, species to form, specific difference to efficient cause, the individual to the final cause.
Everything exists either by itself (_per se_) or in something else. Matter exists by itself, form exists in something else, in matter. Matter is potentially substance; after it assumes a form it becomes actual substance. In reality there is no matter without form, but in thought we can remove the form and leave the matter.
Substance may be described as that which bears opposite and changing qualities. No substance can be the opposite of another substance through its substantiality, but through its accidents; for opposition resides in quality. Matter receiving form is substance. Absolute substance is simple and spiritual, for it cannot be perceived through the five senses. When the philosophers say that all body is substance, and that the individual is a substance, they use substance in contradistinction to accident, meaning that the individual exists by itself, and needs not another for its existence, unlike accidents, which must have something to exist in.
This absolute substance, which is simple and spiritual, seems to be identical with Gabirol's "substantia quæ sustinet decem prædicamenta," the substance which supports the ten categories. Gabirol means by it that which remains of a corporeal substance when we take away from it everything that qualifies it as being here or there, of a particular nature or size, in a given relation, and so on.