His belief as to the relationship of this soul to material things is a difficult and complicated subject which would take us far beyond the topics included in biological writings to-day, but he tells us that ‘there is a class of existent things which we call substance, including under that term, firstly, matter, which in itself is not this nor that; secondly, shape or form, in virtue of which the term this or that is at once applied; thirdly, the whole made up of matter and form. Matter is identical with potentiality, form with actuality,’ the soul being, in living things, that which gives the form or actuality. ‘Of natural bodies’, he continues, ‘some possess life and some do not: where by life we mean the power of self-nourishment and of independent growth and decay’.[45] It should here be noted that in the Aristotelian sense the ovum is not at first a living thing, for in its earliest stage and before fertilization it does not possess soul even in its most elementary form. [45] _De anima_, ii. 1, ii. ‘The term life is used in various senses, and, if life is present in but a single one of these senses, we speak of a thing as living. Thus there is intellect, sensation, motion from place to place and rest, the motion concerned with nutrition, and, further, [there are the processes of] decay and growth,’ all various meanings or at least exhibitions of some form of life. Hence even ‘plants are supposed to have life, for they have within themselves a faculty and principle whereby they grow and decay.... They grow and continue to live so long as they are capable of absorbing nutriment. This form of life can be separated from the others ... and plants have no other faculty of soul at all,’ but only this lowest vegetative soul. ‘It is then in virtue of this principle that all living things live, whether animals or plants. But it is sensation which primarily constitutes the animal. For, provided they have sensation, even those creatures that are devoid of movement and do not change their place are called animals.... As the nutritive faculty may exist without touch or any form of sensation, so also touch may exist apart from other senses.’[46] Apart from these two lower forms of soul, the _vegetative_ or nutritive and reproductive and the _animal_ or sensitive, stands the _rational_ or intellectual soul peculiar to man, a form of soul with which we shall here hardly concern ourselves.[47] [46] _De anima_, ii. 2, ii; 413ᵅ 22. [47] The question of Aristotle’s meaning in connexion with this topic, of primary importance for all thought, has a vast literature. An authoritative work is R. D. Hicks, _Aristotle, De anima_, Cambridge, 1907.