Formal Cause

If the cause […] is a part of the causatum, in the sense that the causatum could not logically be without the cause, it is called an internal cause; otherwise, it is called an external cause. If the cause is of the nature of an individual thing or fact, and the other factor requisite to the necessitation of the causatum is a general principle, I would call the cause a minor, or individuating, or perhaps a physical cause. If, on the other hand, it is the general principle which is regarded as the cause and the individual fact to which it is applied is taken as the understood factor, I would call the cause major, or defining, or perhaps a psychical cause. [—] The defining internal cause is called the formal cause, or form.