Of all the sciences, it has been remarked, that which owes most to Aristotle is the natural history of animals. Not only was he acquainted with numerous species, he also described them according to a comprehensive and luminous method, which perhaps none of his successors have approached; arranging the facts observed, not according to the species, but according to the organs and functions, which affords the only means of establishing comparative results. It may in fact be said, that besides being the oldest author on comparative anatomy whose writings we possess, he was likewise one of those who have treated that part of natural history with most genius, and best deserves to be taken as a model. The principal divisions which are still adopted by naturalists in the animal kingdom are those of Aristotle, and he proposed some which have been resumed after having been unjustly rejected. If we examine the foundation of these great labours, we shall find that they all rest on the same method, which is itself derived from the theory respecting the origin of general ideas. He always observes facts with attention, compares them with great precision, and endeavours to discover the circumstances in which they agree. His style, moreover, is suited to his method: simple, precise, unstudied, and calm, it seems in every respect the reverse of Plato's; but it has also the merit of being generally clear, except in some places where his ideas themselves were not so.[C] In one of his treatises, Aristotle divides natural bodies into those possessing life, and those destitute of that principle,--into animate and inanimate. He considers soul as the vital energy or vivifying principle common to all organized bodies; but distinguishes in it three species. Thus, in plants there is a vegetative, in animals a vegetative and a sentient, in man a vegetative, a sentient, and a rational soul. The functions of nutrition and generation in plants and animals he attributes to the vegetative soul; sense, voluntary motion, appetite, and passion, to the sentient soul; the exercise of the intellectual faculties, to the rational soul.