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Rh commences with inanimate things (which, whatever their character, he distinguishes broadly from whatever has life) and then passes to plants, which he distinguishes from animal bodies, by being non-irritable—incapable, that is, of contracting any of their solids, suddenly and repeatedly, while animal bodies are, on the contrary, endowed with contractile power. Cuvier observes, that "living and organised beings have, from the earliest times, been subdivided into animated beings—beings, that is, which are sentient and moveable, and beings which are inanimate; and as these are neither sentient nor moveable, they are reduced to the common faculty of vegetation or nutrition." It is not necessary, then, as Aristotle remarks, that all living beings should be sentient.