But that which constitutes the specific reality (forma) of a human body is, that its parts communicate their several motions one to another in a certain fixed proportion (Def.

before Lemma iv.

after II.

xiii.).
Therefore, whatsoever brings about the preservation of the proportion between motion and rest, which the parts of the human body mutually possess, preserves the specific reality of the human body, and consequently renders the human body capable of being affected in many ways and of affecting external bodies in many ways; consequently it is good (by the last Prop.).

Again, whatsoever brings about a change in the aforesaid proportion causes the human body to assume another specific character, in other words (see Preface to this Part towards the end, though the point is indeed self--evident), to be destroyed, and consequently totally incapable of being affected in an increased numbers of ways; therefore it is bad.