Here, then, you have a further definition of what nature is as applied to substances alone. This definition comprises also the definition of substance. For if the word nature signifies substance, when once we have defined nature we have also settled the definition of substance. But if we neglect incorporeal substances and confine the name nature to corporeal substances so that they alone appear to possess the nature of substance--which is the view of Aristotle and the adherents both of his and various other schools--we shall define nature as those do who have only allowed the word to be applied to bodies. Now, in accordance with this view, the definition is as follows: "Nature is the principle of movement properly inherent in and not accidentally attached to bodies." I say "principle of movement" because every body has its proper movement, fire moving upwards, the earth moving downwards. And what I mean by "movement properly inherent and not accidentally attached" is seen by the example of a wooden bed which is necessarily borne downward and is not carried downward by accident. For it is drawn downward by weight and heaviness because it is of wood, i.e. an earthly material. For it falls down not because it is a bed, but because it is earth, that is, because it is an accident of earth that it is a bed; hence we call it wood in virtue of its nature, but bed in virtue of the art that shaped it. Nature has, further, another meaning according to which we speak of the different nature of gold and silver, wishing thereby to point the special property of things; this meaning of nature will be defined as follows: "Nature is the specific difference that gives form to anything." Thus, although nature is described or defined in all these different ways, both Catholics and Nestorians firmly hold that there are in Christ two natures of the kind laid down in our last definition, for the same specific differences cannot apply to God and man. II. Sed de persona maxime dubitari potest, quaenam ei definitio possit aptari. Si enim omnis habet natura personam, indissolubilis nodus est, quaenam inter naturam personamque possit esse discretio; aut si non aequatur persona naturae, sed infra terminum spatiumque naturae persona subsistit, difficile dictu est ad quas usque naturas persona perueniat, id est quas naturas conueniat habere personam, quas a personae uocabulo segregari. Nam illud quidem manifestum est personae subiectam esse naturam nec praeter naturam personam posse praedicari. Vestiganda sunt igitur haec inquirentibus hoc modo.