NOVUM ORGANUM 153 



understood, when taken as the genus of heat: it must not 

 be thought that heat generates motion, or motion heat 

 (though in some respects this be true), but that the very 

 essence of heat, or the substantial self 34 of heat, is motion 

 and nothing else, limited, however, by certain differences 

 which we will presently add, after giving some cautions for 

 avoiding ambiguity. 



Sensible heat is relative, and regards man, not universe; 

 and is rightly held to be merely the effect of heat on animal 

 spirit. It is even variable in itself, since the same body (in 

 different states of sensation) excites the feeling of heat and 

 of cold; this is shown by Inst. 41, Tab. 3. 



Nor should we confound the communication of heat or 

 its transitive nature, by which a body grows warm at the 

 approach of a heated body, with the form of heat; for heat 

 is one thing and heating another. Heat can be excited by 

 friction without any previous heating body, and, therefore, 

 heating is excluded from the form of heat. Even when heat 

 is excited by the approach of a hot body, this depends not 

 on the form of heat, but on another more profound and com 

 mon nature; namely, that of assimilation and multiplication, 

 about which a separate inquiry must be made. 



The notion of fire is vulgar, and of no assistance; it is 

 merely compounded of the conjunction of heat and light in 

 any body, as in ordinary flame a nd red-hot substances. 



Laying aside all ambiguity, therefore, we must lastly con 

 sider the true differences which limit motion and render it 

 the form of heat. 



I. The first difference is, that heat is an expansive 

 motion, by which the body strives to dilate itself, and to 



34 &quot;Quid ipsum,&quot; the TO rl %v elvai o f Aristotle. 



