It is not identical with the body; but as form, proportion (λόγος), activity (ἐνέργεια), essence (τὸ τί ἠν εἰναι), it is related to the body, mere matter (ὕλη), and potentiality (δύναμις) in just the same way as the seal is related to the wax; and the body is the instrument whereby it effects its purposes; though subsequent in time, it is prior in thought to the body, as all activities are to the materials with which they operate. At the same time neither it nor its parts are separable from the body, with the exception, possibly, of mind (νου̑ς); it is indeed the actual or possible functioning of the body, like the seeing of the eye or the cutting of the axe, and with the disappearance of the capacity of this functioning the soul itself also perishes. Lastly, it is a cause (ἀρχὴ καὶ αἰτία) in a triple sense: first, as the source of motion; secondly, as that for the sake of which the body exists; and thirdly, as its essence (οὐσία) or formal cause. The soul or life is of several kinds, which form together an ascending series each member of which is necessarily involved in those above it. The lowest is the nutritive soul (θρεπτική), found in all living things, and the only soul possessed by plants. It is defined as motion in respect of nutrition, decay, and growth, processes which involve alteration (ἀλλοίωσις) in the body; and its functions (ἔργα) are to utilize food for the maintenance and reproduction of the form of the body, and to control and limit growth. The second is the perceptive soul (αἰσθητική), the possession of which distinguishes animals from plants. This also is a kind of alteration (ἀλλοίωσις τίς) and consists in being moved and affected. The fundamental and indispensable perception is touch (ἁφή), for it is concerned in the acquisition of the food. It is invariably present: the others may or may not, some or all, coexist with it. Thirdly, some animals are possessed of a capacity for locomotion, and the performance of this function requires again a special kind of soul. Lastly, there is the reasoning soul (διανοητικά) or mind (νου̑ς). This is found in man alone, unless there be other beings similar to him, or even nobler than he. Mind alone is eternal and separable from the body. Though the observation and experiment of modern science would doubtless find much to alter in the details of these simple definitions, yet it must be conceded that, by what is certainly a most fortunate guess if it is not the most wonderful insight, Aristotle has laid his finger on the cardinal point of modern physiological doctrine. For, putting aside for the moment the mental faculties, it is here laid down in the clearest manner that not only the functions of growth and decay, nutrition, and reproduction, but also the capacity of responding to stimuli are to be ultimately resolved into some kind of movement of the particles of which the body is composed. Life, in short, as we might say with Virchow, is a mode of motion.