250 SHEiLAii o'flynn brennan 



Such, in brief, is Aristotle's delineations of various meanings 

 of nature.^ It is to be noted that all the senses except the 

 last include a relation to movement, and this last Aristotle is 

 careful to set off from the others by indicating that it involves 

 " an extension of meaning." Nature, then, is to be seen prin- 

 cipally as a source of movement in things. 



Before proceeding to the Physics, it is of interest to note 

 St. Thomas' introductory comment upon this chapter of the 

 Metaphysics: " Though the consideration of [nature] does not 

 seem to belong to first philosophy, but rather to natural phi- 

 losophy, [Aristotle] nevertheless distinguishes the meaning of 

 this word here [in first philosophy] because nature according 

 to one of its senses is said of every substance." In this passage 

 Aquinas is obviously referring to the extended meaning of 

 nature; the other meanings, then, would apparently belong 

 properly to philosophy of nature. 



This is precisely what we find when we turn to the Physics, 

 Book II, Chapter 1.*' Let us briefly review Aristotle's pro- 

 cedure: First he points out that things which exist by nature 

 are seen to differ from artifacts in that the former have within 



^ These, of course, are not the only ways in which Aristotle uses the word. For 

 a comprehensive list of the meanings of <pv<ns in Aristotle, see H. Bonitz, iTidez 

 Aristotdicus (Graz, 1955) . 



" St. Thomas establishes the meaning of nature in philosophy of nature right at 

 the beginning of his commentary of the Physics. In lesson 1, Bk I of his com- 

 mentary he shows the subject of the science of nature to be that which depends on 

 matter for both its being and its definition, as distinct from mathematical en- 

 tities and the subject of metaphysics. St. Thomas then explains: " Because every- 

 thing that has matter is mobile, consequently the subject of natural philosophy is 

 mobile being. For natural philosophy is about natural things, which are those 

 whose principle is nature. Now nature is the principle of motion and rest in that 

 in which it is. Natural science, therefore, is about those things which have in 

 themselves a principle of motion." Natural being is here clearly identified with 

 mobile being and mobile being with sensible material being. Nature has the 

 meaning, not of what the thing is or the essence simply, but of principle of 

 movement or change, such as movement according to place or even of generation. 

 Movement, inasmuch as it is given as implying matter, is obviously to be understood 

 in the strict sense, as actus imperfecti, and not according to an extended meaning 

 which could also include any type of operation, even thought. Otherwise the 

 natural and mobile being would not necessarily be a material being, as it is explicitly 

 stated to be, since there are operations which do not presuppose matter. 



