MEDICINE IN MODERN TIMES. 721 



In this part of my address I propose to consider, mainly by the 

 light of recently attained biological results, the value of two great 

 rules for the conduct of the understanding, each of which has a 

 legitimate sphere of application, but the former of which enjoySj 

 it seems to me, more and the latter less than its deserved promi- 

 nence. The first of these two regulative principles has received 

 the endorsement of Newton, and it stands as his first ' Reffula 

 Philosophandi,' at the commencement of the Third Book of the 

 Principia. It was known in the days of the schoolmen as the 

 ' Razor of Occam,' and in later days it has been styled the ' Law of 



Parsimony' or 'Economy.' Newton enunciates it as follows: 



'Causas rerum naturalium non plures admitti debere, quam quae 

 et verae sint et earum phaenomenis explicandis sufRciant. Dicunt 

 utique philosophi : "Natura nihil agit frustra;" et "Frustra fit 

 per plura quod fieri potest per pauciora." Natura enim simplex est 

 et rerum causis superfluis non luxuriat.' I know that this Regula 

 has great influence on the minds of many biologists, and I believe 

 that this its influence is by no means always for good. This is not 

 a subject in which authorities ought to count for much; but I may 

 say that, while the names of Aristotle, Malebranche, Maupertuis, 

 St. Hilaire, Goethe, Bichat, and Dugald Stewart may be quoted in 

 approval of this rule, the names of Bacon. Mill, and De Candolle 

 may be brought forward by those who repudiate it or curtail its 

 application. Our motto, however, is, *Nullius addictus jurare in 

 verba magistri ;' and our business is to ask, not what men have laid 

 down, but how Nature operates. Can a phaenomenon have more 

 than one cause, or can it not ? Is it possible, for example, and to 

 put the question in a concrete and most practically interesting 

 point of view at once, that a fever which we know can spread by 

 infection or contagion, can also originate spontaneously? I rather 

 incline, though but doubtfully, and after an imperfect examination 

 of imperfect data, to anticipate that a negative answer to this latter 

 question will turn out some day to be the true one ; but I do not 

 know that there is anything in the analogy of Nature to compel us 

 to incline towards this negative conclusion a priori. Such a phae- 

 nOmenon, at all events, as a living animal, is often enough produced 

 by two or more distinct processes, within the limits of the same 

 species: as, for example, from ova of different character, summer 

 ova or winter ova, impregnated or unimpregnated ova; by fission 



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