Not a 'soul' in the generally accepted Western use of the term, this 'soul', he says, is a life force that is indivisible from the organism that possesses it. The essay is divided into three books. Presenting his concept in book I, he further describes the structure of the 'souls' of plants, animals and humans in book II and book III.
In 'The Parva Naturalia' ('Little Physical Treatises'), Aristotle continues his investigation into the biology of life and the links between body and 'soul'. It consists of seven essays: 'Sense and Sensibilia', 'On Memory', 'On Sleep', 'On Dreams', 'On Divination in Sleep', 'On Length and Shortness of Life', 'On Youth', 'Old Age', 'Life and Death' and 'Respiration'.