INTRODUCTION. 45 



commonly, not formed by a single act ; they are not completed by 

 the discovery of one great principle. On the contrary, they consist in 

 a long-continued advance; a series of changes; a repeated progress 

 from one principle to another, different and often apparently contradic- 

 tory. Now, it is important to remember that this contradiction is 

 apparent only. The principles which constituted the triumph of the 

 preceding stages of the science, may appear to be subverted and ejected 

 by the later discoveries, but in fact they are (so far as they were true) 

 taken up in the subsequent doctrines and included in them. They 

 continue to be an essential part of the science. The earlier truths are 

 not expelled but absorbed, not contradicted but extended; and the 

 history of each science, which may thus appear like a succession of 

 revolutions, is, in reality, a series of developments. In the intellectual, 

 as in the material world, 



Omnia mutantur nil interit 



Nee manet nt fuer.it nee formas servat easdem, 

 Sed tamen ipsa eadein est. 



All changes, naught is lost ; the forms are change:!, 

 And that which has been is not what it was, 

 Yet that which has been is. 



Nothing which was done was useless or unessential, though it ceases 

 to be conspicuous and primary. 



Tims the final form of each science contains the substance of each 

 of its preceding modifications; and all that was at any antecedent 

 period discovered and established, ministers to the ultimate develop- 

 ment of its proper branch of knowledge. Such previous doctrines may 

 require to be made precise, and definite, to have their superfluous and 

 arbitrary portions expunged, to be expressed in new language, to be 

 taken up into the body of science by various processes ; but they do 

 not on such accounts cease to be true doctrines, or to form a portion 

 of the essential constituents of our knowledge. 



O 



Terms record Discoveries* The modes in which the earlier truths 

 of science are preserved in its later forms, are indeed various. From 

 being asserted at first as strange discoveries, such truths come at last 

 to be implied as almost self-evident axioms. They are recorded by 

 Borne familiar maxim, or perhaps by some new word or phrase, which 

 becomes part of the current language of the philosophical world; and 

 thus asserts a principle, while it appears merely to indicate a transient 



Concerning Technical Twins, see rhiljsonly, book i. ch. S. 



