How does Spinoza define a body? A body, of whatever kind, is defined by Spinoza in two simultaneous ways. In the first place, a body, however small it may be, is composed of an infinite number of particles; it is the relations of motion and rest, of speeds and slownesses between particles, that define a body, the individuality of a body. Secondly, a body affects other bodies, or is affected by other bodies; it is this capacity for affecting and being affected that also defines a body in its individuality. These two propositions appear to be very simple; one is kinetic and the other dynamic. But if one truly installs oneself in the midst of these propositions, if one lives them, things are much more complicated and one finds that one is a Spinozist before having understood why.
That's an excerpt from Gilles Deleuze, writing in Spinoza and Us. This text, so often under appreciated, both in its scope and style, points to the construction of a new type of ethics.