In his very brief, yet very significant discussion of the nature of physical bodies, Spinoza describes a hierarchy, or a series continuously increasing in degree of complexity. The simplest bodies are distinguished from one another only by their state of motion, but any contiguous group, which transmit to one another a constant proportion of motion and rest, may be regarded as a single individual; and a group of such groups, on similar conditions, constitutes a more complex unity. The series continues indefinitely until the physical universe is seen as one single whole governed by a principle of organization which determines the proportion of motion and rest transmitted from one to another of its internally distinguishable parts.