 Bodies are individuated one from the other, not by reason of substance, but rather by reason of motion and rest (IIp13sl1), individual identity through time and change being a matter of the dynamic maintenance of a distinctive ratio of motion and rest of a body's parts (IIp13s Def.). The human body is simply a particularly complex individual body, capable of maintaining its distinctive structure through a wide variety of externally imposed modifications, and capable of forming corporeal images of those bodies that affect it. An affect that increases the body's ability to maintain its distinctive ratio is paralleled in the mind by a modification that increases that mind's power of thought, and the passage from lesser to greater power is joy (IIIp11s, Affect Def. II). Modifications that decrease the power of a body to maintain its individual ratio are paralleled by modifications decreasing the mind's power of thought, and such passages are sadness (IIIp11s, Affect Def. III). Further, individuals, both minds and bodies, strive to persevere in their existence as far as they can; that is to say, they strive to increase the power by which they maintain their distinctive natures (IIIp6). Bodies, then, have essences, which are powers of striving (IIIp7). The causal interactions in which a body participates that are attributable to the action of that body increase its power of perseverance; it is in contrast passive with respect to those that diminish its power. Evidently, for Spinoza, strict necessitarian determinism is consistent with a genuine distinction between action and passion, between doing and suffering an act.