512 THE PRINCIPLES OF SCIENCE. [CHAP. 



to those conditions. Hence, as Boscovich truly said, we 

 are to understand by hypotheses " not fictions altogether 

 arbitrary, but suppositions conformable to experience or 

 analogy." It follows that every hypothesis worthy of 

 consideration must suggest some likeness, analogy, or 

 common law, acting in two or more things. If, in order 

 to explain certain facts, a, a', a", &c., we invent a cause A, 

 then we must in some degree appeal to experience as to 

 the mode in which A will act. As the laws of nature are 

 not known to the mind intuitively, we must point out 

 some other cause, B, which supplies the requisite notions, 

 and all we do is to invent a fourth term to an analogy. 

 As B is to its effects &, &', b", &c., so is A to its effects a, 

 a', a", &c. When we attempt to explain the passage of 

 light and heat radiations through space unoccupied by 

 matter, we imagine the existence of the so-called ether. 

 But if this ether were wholly different from anything 

 else known to us, we should in vain try to reason about it. 

 We must apply to it at least the laws of motion, that is 

 we must so far liken it to matter. And as, when applying 

 those laws to the elastic medium air, we are able to infer 

 the phenomena of sound, so by arguing in a similar manner 

 concerning ether we are able to infer the existence of light 

 phenomena corresponding to what do occur. All that we 

 do is to take an elastic substance, increase its elasticity 

 immensely, and denude it of gravity and some other 

 properties of matter, but we must retain sufficient likeness 

 to matter to allow of deductive calculations. 



The force of gravity is in some respects an incompre- 

 hensible existence, but in other respects entirely con- 

 formable to experience. We observe that the force is 

 proportional to mass, and that it acts in entire indepen- 

 dence of other matter which may be present or intervening. 

 The law of the decrease of intensity, as the square of the 

 distance increases, is observed to hold true of light, sound, 

 and other influences emanating from a point, and spreading 

 uniformly through space. The law is doubtless connected 

 with the properties of space, and is so far in agreement 

 with our necessary ideas. 



It may be said, however, that no hypothesis can be so 

 much as framed in the mind unless it be more or less 

 conformable to experience. As the material of our ideas 



