An Introduction to the Ten Categories

Beings in the world, that is, outside the mind, exist not primarily as numbers, or atoms, or waves, or particles, but as entities or things. In other words, beings exist primarily as substances, according to Aristotle. A substance exists per se, that is, in itself (independently). But there are secondary modes of being that do not exist per se, but rather exist dependently. Their mode of being is to exist in, that is, in something more primary, such as substance. For example, there is no such thing as red, or 50 pounds. There are only red things, or 50 pound things. Nor is there such a thing as equality. Rather, there are things that are equal, either in size, or in quality, etc. These secondary modes of being are called accidents, from the Latin , which means to inhere in.

Substance (principal mode of being): Such as water, gold, a tree, a dog, or a human being.

Accidents (secondary modes of being)

The substance is that in which the accidents inhere; it is the substratum of the accidental modes of being. Accidents actuate the substance in an accidental way, that is, in a way that does not change the substance itself. The first accident quantifies a substance. But quantity is a distinct mode of being than substance. To inquire of a substance is not necessarily to inquire of its quantity. Moreover, quantity can change while the substance endures. A human being, for instance, was at one time smaller than your hand. He increased in quantity and is now six feet ,five inches, and weighing three hundred pounds. But even though his quantity changed, he (the substance) remained the same substance. If substance were to be identified with quantity, then a change in quantity would amount to a change in substance.

Similarly, to inquire of a thing's quality is not necessarily to inquire of its quantity. Quality is a distinct mode of being than quantity and substance, and so on with all the other remaining accidents.

Corresponding to the two modes of being (substance and accident) are two types of changes: substantial and accidental. An example of an accidental change is a change in quantity (a substance becomes larger), or a change in quality (a substance changes from bitter to sweet). In these cases, it is the substance that remains the same throughout the change, that is, the substance is the subject that underlies the change.

A substantial change is a change in substance. For example, the change from wood to ash, living organism to rotting corpse, or hydrogen and oxygen to water, or sodium and chlorine to salt, or sperm and ovum to conceptus, etc.

For every change, we speak of two terminals, namely the terminus a quo and the terminus ad quem.

Terminus a quo refers to the "terminal from which" (from which the change commences), while terminus ad quem refers to the "terminal towards which" (at which the change ends).

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