Aristotle thinks of living things as food burners: they nourish themselves, and so, in some cases, possess the capacity for higher living functions such as perceiving. Their burning must be balanced, if it is to continue - and one way they do this is through breathing. Nonetheless, all such burning naturally develops and declines, thus describing the life span of the being concerned. Dr Kings' treatment provides a detailed reading of the end of the "Parva Naturalia" ("On the Length and Shortness of Life", "On Youth, Old Age, Life and Death", including "On Breathing"). He shows how the investigation into life begun in the "De Anima" is completed in the "Parva Naturalia", culminating in definitions of the stages of the life cycle, from generation of a new living thing up to death, using the activity of nutrition.