752 THE PEINCIPLES OF SCIENCE. [CHAP. 



This is doubtless a mere speculation, practically in- 

 capable of verification by observation, and almost free 

 from restrictions afforded by present knowledge. We 

 might attribute various shapes to the adamantine medium, 

 and the consequences would be various. But there is this 

 value in such speculations, that they draw attention to the 

 finiteness of our knowledge. We cannot deny the possible 

 truth of such an hypothesis, nor can we place a limit to 

 the scientific imagination in the framing of other like 

 hypotheses. It is impossible, indeed, to follow out our 

 scientific inferences without falling into speculation. If 

 heat be radiated into outward space, it must either proceed 

 ad infinitmn, or it must be stopped somewhere. In the 

 latter case we fall upon Rankine's hypothesis. But if the 

 material universe consist of a finite collection of heated 

 matter situated in a finite portion of an infinite adamantine 

 medium, then either this universe must have existed for a 

 finite time, or else it must have cooled down during the 

 infinity of past time indefinitely near to the absolute zero 

 of temperature. I objected to Lucretius' argument against 

 the destructibility of matter, that we have no knowledge 

 whatever of the laws according to which it would undergo 

 destruction. But we do know the laws according to which 

 the dissipation of heat appears to proceed, and the con- 

 clusion inevitably is that a finite heated material body 

 placed in a perfectly cold infinitely extended medium 

 would in an infinite time sink to zero of temperature. 

 Now our own world is not yet cooled down near to zero, 

 so that physical science seems to place us in the dilemma 

 of admitting either the finiteness of past duration of the 

 world, or else the finiteness of the portion of medium in 

 which we exist. In either case we become involved in 

 metaphysical and mechanical difficulties surpassing our 

 mental powers. 



The Divergent Scope for New Discovery. 



In the writings of some recent philosophers, especially 

 of Auguste Comte, and in some degree John Stuart Mill, 

 there is an erroneous and hurtful tendency to represent 

 our knowledge as assuming an approximately complete 

 character. At least tnese and many other writers fail to 



