HYPOTHESES. 349 



propei'ties of the different kinds of substances, but depend on conditions 

 capable of being superinduced upon all substances ; since there is no sub- 

 stance which can not, according to the kind of light thrown upon it, be 

 made to assume almost any color; and since almost every change in the 

 mode of aggregation of the particles of the same substance is attended 

 with alterations in its color, and in its optical properties generally. 



The really weak point in the attempts which have been made to account 

 for colors by the vibrations of a fluid, is not that the attempt itself is un- 

 philosophical, but that the existence of the fluid, and the fact of its vibra- 

 tory motion, are not proved, but are assumed, on no other ground than 

 the facility they are supposed to afford of explaining the phenomena. And 

 this consideration leads to the important question of the proper use of 

 scientific hypotheses, the connection of which with the subject of the ex- 

 planation of the phenomena of nature, and of the necessary limits to that 

 explanation, need not be pointed out. 



§ 4. An hypothesis is any supposition which we make (either without 

 actual evidence, or on evidence avowedly insufiicient) in order to endeavor 

 to deduce from it conclusions in accordance with facts which are known to 

 be real; under the idea that if the conclusions to which the hypothesis 

 leads are known truths, the hypothesis itself either must be, or at least is 

 likely to be, true. If the hypothesis relates to the cause or mode of pro- 

 duction of a phenomenon, it will serve, if admitted, to explain such facts as 

 are found capable of being deduced from it. And this explanation is the 

 purpose of many, if not most hypotheses. Since explaining, in the scien- 

 tific sense, means resolving a uniformity which is not a law of causation, 

 into the laws of causation from which it results, or a complex law of causa- 

 tion into simpler and more general ones from which it is capable of being 

 deductively inferred, if there do not exist any known laws which fulfill 

 this requirement, we may feign or imagine some which would fulfill it; 

 and this is making an hypothesis. 



An hypothesis being a mere supposition, there are no other hmits to 

 hypotheses than those of the human imagination; we may, if we please, 

 imagine, by way of accounting for an effect, some cause of a kind utterly 

 unknown, and acting according to a law altogether fictitious. But as hy- 

 potheses of this sort would not have any of the plausibility belonging to 

 those which ally themselves by analogy with known laws of nature, and be- 

 sides would not supply the want which arbitrary hypotheses are generally 

 invented to satisfy, by enabling the imagination to represent to itself an 

 obscure phenomenon in a familiar light, there is probably no hypothesis 

 in the history of science in which both the agent itself and the law of its 

 operation were fictitious. Either the phenomenon assigned as the cause is 

 real, but the law according to which it acts merely supposed ; or the cause 

 is fictitious, but is supposed to produce its effects according to laws similar 

 to those of some known class of phenomena. An instance of the first kind 

 is afforded by the different suppositions made respecting the law of the 

 planetary central force, anterior to the discovery of the true law, that the 

 force varies as the inverse square of the distance; which also suggested 

 itself to Newton, in the first instance, as an hypothesis, and was verified 

 by proving that it led deductively to Kepler's laws. Hypotheses of the 

 second kind are such as the vortices of Descartes, which were fictitious, 

 but were supposed to obey the known laws of rotatory motion; or the 

 two rival hypotheses respecting the nature of light, the one ascribing 



