624 THE PRINCIPLES OF SCIENCE. |CH. 



known laws, or when they lead us to mix up discrete things. 

 Because we speak of vital force we must not assume that it 

 is a really existing physical force like electricity ; we do not 

 know what it is. We have no right to confuse Encke's 

 supposed resisting medium with the basis of light without 

 distinct evidence of identity. The name protoplasm, now 

 so familiarly used by physiologists, is doubtless legitimate 

 so long as we do not mix up different substances under it, 

 or imagine that the name gives us any knowledge of the 

 obscure origin of life. To name a substance protoplasm 

 no more explains the infinite variety of forms of life which 

 spring out of the substance, than does the vital force which 

 may be supposed to reside in the protoplasm. Both ex- 

 pressions are mere names for an inexplicable series of 

 causes which out of apparently similar conditions pro- 

 duce the most diverse results. 



Hardly to be distinguished from descriptive hypotheses 

 are certain imaginary objects which we frame for the 

 ready comprehension of a subject. The mathematician, 

 in treating abstract questions of probability, finds it con- 

 venient to represent the conditions by a concrete hypo- 

 thesis in the shape of a ballot-box. Poisson proved the 

 principle of the inverse method of probabilities by ima- 

 gining a number of ballot-boxes to have their contents 

 mixed in one great ballot-box (p. 244). Many such 

 devices are used by mathematicians. The Ptolemaic 

 theory of cycles and epi-cycles was no grotesque and use- 

 less work of the imagination, but a perfectly valid mode 

 of analysing the motions of the heavenly bodies ; in reality 

 it is used by mathematicians at the present day. Newton 

 employed the pendulum as a means of representing the 

 nature of an undulation. Centres of gravity, oscillation, 

 &c., poles of the magnet, lines of force, are other imaginary 

 existences employed to assist our thoughts (p. 364). Such 

 devices may be called Representative Hypotheses, and they 

 are only permissible so far as they embody analogies. 

 Their further consideration belongs either to the subject 

 of Analogy, or to that of language and representation, 

 founded upon analogy. 



