11. I. 21 



property for simple prehension.^o For this reason the active or 

 executive parts of the body are compounded out of bones, sinews, 

 flesh, and the like, but not these latter out of the former. 



So far, then, as has yet been stated, the relations between these 

 two orders of parts are determined by a final cause. We have 

 however to inquire whether necessity may not also have a share 

 in the matter ; and it must be admitted that these mutual relations 

 could not from the very beginning have possibly been other than 

 they are. For heterogeneous parts can be made up out of homo- 

 geneous parts, either from a plurality of them, or from a single 

 one, as is the case with some of the viscera, which varying in 

 configuration are yet, to speak broadly, formed from a single 

 homogeneous substance ; but that homogeneous substances should 

 be formed out of a combination of heterogeneous parts is clearly 

 an impossibility. For these causes, then, some parts of animals are 

 simple and homogeneous, while others are composite and hetero- 

 geneous ; and dividing the parts into the active or executive 

 and the sensitive, each one of the former is, as before said, hetero- 

 geneous, and each one of the latter homogeneous. For it is in 

 homogeneous parts alone that sensation can occur, as the following 

 considerations show. 



Each sense is confined to a single kind or order of sensibles, 

 and its organ must therefore be such as to admit the action of 

 that kind, and [of no other. It must therefore have the properties 

 of that kind and of no other ; for] that which is endowed with 

 a property in posse is acted upon by that which has the like 

 property in esse}^ The one-ness of its sensibles implies then the 

 one-ness or homogeneity of the sense-organ. Thus it is that while 

 no physiologists ever dream of saying of the hand or face or other 

 such part that one is earth, another water, another fire, they couple 

 each separate sense-organ with a separate element, asserting this 

 one to be air and that other to be fire.'^ 



Sensation then is confined to the simple or homogeneous parts. 

 But, as might reasonably be expected, the organ of touch, though 

 still homogeneous, is yet the least simple of all the sense-organs. 

 For touch more than any other sense appears to be correlated to 

 several distinct kinds of objects, and to recognise more than one 

 category of contrasts, heat and cold, for instance, solidity and 

 fluidity, and other similar oppositions.^^ Accordingly the organ 

 which deals with these varied objects is of all the sense-organs 

 647 a. 



