264 SHEiLAii o'flynn brennan 



furthermore, that the principle of sensation in animals as such 

 is not a source of movement in the strict sense and the principle 

 of intellection in men has even less reason to be considered as 

 a cause of movement. This last extension of meaning, however, 

 does not coincide with the sense that is broad enough to include 

 even the essence of immaterial substance as root of spiritual 

 activity. It is still proper to philosophy of nature, for though 

 the operation in question as spiritual is not movement, still as 

 the operation of a form in a body, it must take place in con- 

 junction with activities that are movements. The rational soul, 

 as a form in matter, cannot effectually be a principle of under- 

 standing unless it be at the same time a principle of move- 

 ment. At this point, however, the extension of the word has 

 reached a limit beyond which the meaning would no longer be 

 proper to philosophy of nature. 



The word nature, consequently, though it has a sense proper 

 to philosophy of nature, admits of a wide variety of meanings 

 even within this science. In the Physics, we have seen, nature 

 is said most obviously of matter and most properly of form, 

 but of both insofar as they are principles of movement in the 

 strict sense. Matter is such a principle by its aptitude for form, 

 thus implying an inclination to being. Form as fulfilling this 

 aptitude, and as an end of generation, is also a principle of 

 movement. Once the natural being is in existence, form is 

 nature as the active principle of movements necessary for its 

 preservation in existence and the attainment of its good in 

 general or of movements contributing to the good of the uni- 



consider the active principle of the generating agent relatively to the generated. 

 Here the active potency is undoubtedly the mover of another. There is a sense, 

 though, in which even this active principle can be called nature, for although it is 

 extrinsic to the particular mobile being that is the product of the generation, never- 

 theless both mover and moved coincide in the same natural species. (Cf. St. 

 Thomas, In VII Metaph., lect. 6, nn. 1386-1393.) What is more, the universe 

 could be considered as a whole having heterogeneous parts acting one upon the 

 other. In this case, the form that constitutes anything as an agent with respect to 

 something else could be considered as a principle intrinsic to the moving whole 

 (although extrinsic to the particular thing it moves) and as such could be called 

 nature. 



