XXIIL] THE USE OF HYPOTHESIS. 519 



in possession of two or more hypotheses which both agree 

 with so many experimental facts as to have great appear- 

 ance of truth. Under such circumstances we have need 

 of some new experiment, which shall give results agreeing 

 with one hypothesis but not with the other. 



Any such experiment which decides between two rival 

 theories may be called an Experimentum Crucis, an 

 Experiment of the Finger Post. Whenever the mind 

 stands, as it were, at cross-roads and knows not which 

 way to select, it needs some decisive guide, and Bacon 

 therefore assigned great importance and authority to in- 

 stances which serve in this capacity. The name given by 

 Bacon has become familiar ; it is almost the only one of 

 Bacon's figurative expressions which has passed into com- 

 mon use. Even Newton, as I have mentioned (p. 507), 

 used the name. 



I do not think, indeed, that the common use of the 

 word at all agrees with that intended by Bacon. Her- 

 schel says that " we make an experiment of the crucial 

 kind when we form combinations, and put in action 

 causes from which some particular one shall be deliberately 

 excluded, and some other purposely admitted." l This, 

 however, seems to be the description of any special ex- 

 periment not made at haphazard. Pascal's experiment 

 of causing a barometer to be carried to the top of 

 the Puy-de-D6me has often been considered as a per- 

 fect experimentum crucis, if not the first distinct one on 

 record ; 2 but if so, we must dignify the doctrine of 

 Nature's abhorrence of a vacuum with the position of a 

 rival theory. A crucial experiment must not simply 

 confirm one theory, but must negative another ; it must 

 decide a mind which is in equilibrium, as Bacon says, 1 

 between two equally plausible views. "When in search 

 of any nature, the understanding comes to an equilibrium, 

 as it were, or stands suspended as to which of two or 

 more natures the cause of nature inquired after should 

 be attributed or assigned, by reason of the frequent and 

 common occurrence of several natures, then these Crucial 

 Instances show the true and inviolable association of one 



1 Discourse on the Study of Natural Philosophy, p. 151. 



2 Ibid. p. 229. 



3 Novum Organum, bk. ii. Aphorism 36. 



