Aristotle's Psychology: On the Soul and Parva Naturalia

that they possess rudimentary sensation,¹ although they are incapable of locomotion, and can be regarded only as belonging to the initial stage of animal development. Nature completes the transition from plant organisms to animals proper by an increased or added activity of the soul, in which are manifested the further phenomena of sensibility, with which desire is associated, and desire demands locomotion. An animal soul is a more complex and more highly developed form of the original life-principle.

While we in modern times, in popular language at least, differentiate the life found in the plant-world from that which is found in the animal-world (though the boundary between these two is not exactly defined) by the obvious distinctions of ‘vegetable’ and ‘animal’ life, Aristotle regards them as fundamentally the same. He looks upon the functions of sensation, locomotion, and conceptual thought as a higher development of the vital principle found in plants. We distinguish between sensation and
Red bracket in left margin alongside the text from "conceptual thought" down to "unitary vital force."

conceptual thought without ascribing them to a different mind, as Plato did; but Aristotle goes further and maintains that not only these, but also the function of nutrition, are due to the same unitary vital force. It is, however, a distinctly marked stage that nature makes in the development of the vital principle when sensation is exceeded and rational thought is reached. This new phenomenon is confined to man, and is the last stage in the evolution of ψυχή psyche. Soul is, therefore, in the opinion of Aristotle, the
Blue pencil checkmark in margin

unity in which the principles of life, sense-perception, and