178 Dr. WHEWELL, ON THE FUNDAMENTAL ANTITHESIS OP PHILOSOPHY. 



22. What the marks are, which distinguish the constant differences of kinds of things (definite 

 marks, selected from among many unessential appearances), and what their definite properties are, 

 when they are so distinguished, are parts of our knowledge to be learnt from observation, by 

 various processes ; for instance, among others, by chemical analysis. We find the differences of 

 bodies, as shown by such analysis, to be of this nature: — that there are various elementary 

 bodies, which, combining in different definite proportions, form kinds of bodies definitely different. 

 But, in arriving at this conclusion, we introduce a new idea, that of Elementary Composition, 

 which is not extracted from the phenomena, but supplied by the mind, and introduced in order 

 to make the phenomena intelligible. That this notion of elementary composition is not supplied 

 bv the chemical phenomena of cumbusfion, mixture, &c. as merely an observed fact, we see from 

 this ; tliat men had in ancient times performed many experiments in which elementary composition 

 was concerned, and had not seen the fact. It never was truly seen till modern times ; and when 

 seen, it gave a new aspect to the whole body of known facts. This idea of elementary composition, 

 then, is supplied by the mind, in order to make the facts of chemical analysis and synthesis 

 intelligible as analysis and synthesis. And this idea being so supplied, there enters into our 

 knowledge along with it a corresponding necessary principle; — That the elementary composition of 

 a body determines its kind and proportions. This is, I say, a principle assumed, as a con- 

 sequence of the idea of composition, not a result of experience ; for when bodies have been divided 

 into their kinds, we take for granted that the analysis of a single specimen may serve to determine 

 the analysis of all bodies of the same kind : and without this assumption, chemical knowledge 

 with regard to the kinds of bodies would not be possible. It has been said that we take only 

 one experiment to determine the composition of any particular kind of body, because we have 

 a thousand experiments to determine that bodies of the same kind have the same composition. 

 But this is not so. Our belief in the principle that bodies of the same kind have the same com- 

 ])osition is not established by experiments, but is assumed as a necessary consequence of the ideas of 

 Kind and of Composition. If, in our experiments, we found that bodies supposed to be of the same 

 kind had not the same composition, we should not at all doubt of the principle just stated, but 

 conclude at once that the bodies were not of the same kind ; — that the marks by which the kinds 

 are distinguished had been wrongly stated. This is what has very frequently happened in the 

 course of the investigations of chemists and mineralogists. And thus we have it, not as an 

 experiential fact, but as a necessary principle of chemical philosophy, that the Elementary Com- 

 position of a body determines its Kind and Properties. 



23. How bodies differ in their elementary composition, experiment must teach us, as we have 

 already said that experiment has taught us. But as we have also said, whatever be the nature 

 of this difference, kinds must be definite, in order that language may be possible: and hence, 

 whatever be the terms in which we are taught by experiment to express the elementary com- 

 position of bodies, the result must be conformable to this principle, That the differences of elementary 

 composition are definite. The law to which we are led by experiment is, that the elements of 

 bodies continue in definite proportions according to weight. Experiments add other laws ; as for 

 instance, that of multiple proportions in different kinds of bodies composed of the same elements; 

 but of these we do not here speak. 



24. We are thus led to see that in our knowledge of mechanics, chemistry, and the like, 

 there are involved certain necessary principles, derived from our ideas, and not from experience. 

 But to this it may be objected, that the parts of our knowledge in which these principles are in- 

 volved has, in historical fact, all been acquired by experience. The laws of motion, the doctrine 

 of definite proportions, and the like, have all become known by experiment and observation ; and 

 so far from being seen as necessary truths, have been discovered by long-continued labours and 

 trials, and through innumerable vicissitudes of confusion, error, and imperfect truth. This is 

 perfectly true: but does not at all disprove what has been said. Perception of external objects 



