MENTAL EVOLUTION AND NECESSARY TRUTHS. 359 



recent times, all mankind were in this state of incapacity with respect 

 to physical axioms ; and the mass of mankind are so still. Various 

 popular notions betray inability to form clear ideas of forces and their 

 relations, or carelessness in thinking, or both. EtFects are expected 

 without causes of fit kinds ; or effects extremely disproportionate to 

 causes are looked for ; or causes are supposed to end without eftects. 

 But though many are thus incapable of grasping physical axioms, it 

 no more foliow^s that physical axioms are not knowable a priori by a 

 developed intelligence, than it follows that there is no necessity in 

 logical relations because many have intellects not developed enough 

 to perceive the necessity. 



" The ultimate physical truth of which clear apprehension is event- 

 ually reached is, that force can neither arise without an equivalent 

 antecedent, nor disappear without an equivalent consequent. Along 

 with power of introspection there comes recognition of the fact that 

 existence cannot be conceived as beginning or ending : the Laws of 

 Thought themselves negative any such mental representation. And 

 if it be asked why this intuition, which all physical axioms indirectly 

 imply, and which is postulate in every physical experiment, is to be 

 taken as authoritative because its negation is inconceivable, the answer 

 is that no argument which sets out to discredit it can do this without 

 logical suicide ; since tliere is no other wai-rant for asserting tlie de- 

 pendence of any conclusion on its premises than the inconceivability 

 of its negation." 



This passage forms part of a revised version of the chapters on 

 Matter, Motion, and Force, which I have contemplated making for 

 this year past. When those chapters were written and stereotyped, 

 in April, 1861 (see Preface), the modern doctrines concerning Force 

 and its transformation were so imperfectly developed, that some of 

 the leading technical words now currently used were not introduced. 

 Tlie reorganization of "First Principles," which I made in 1867, for 

 tlie purpose of more truly presenting the general Tlieory of Evolution, 

 did not implicate these chapters, and I believe I did not even re-read 

 them : the stereotype plates, in common with those of many other 

 chapters, witli the numberings of pages and sections altered, were 

 used afresh, and continue still to stand as they originally did. But 

 while now rectifying defects of statement which it was scarcely pos- 

 sible to avoid thirteen years ago, I find no reason for changing the 

 essential conception set forth in those chapters ; nor is tlie need for 

 changing it suggested to me by those on whose judgments I have the 

 best reasons for relying. 



