40 SCIENTIFIC INVESTIGATION 



not extend beyond the limits of our faculties; while, even 

 within those limits, we cannot be certain that any ob- 

 servation is absolutely exact and exhaustive. Hence it 

 follows that any given generalisation from observation 

 may be true, within the limits of our powers of observa- 

 tion at a given time, and yet turn out to be untrue, when 

 those powers of observation are directly or indirectly en- 

 larged. Or, to put the matter in another way, a doc- 

 trine which is untrue absolutely, may, to a very great 

 extent, be susceptible of an interpretation in accordance 

 with the truth. At a certain period in the history of 

 astronomical science, the assumption that the planets 

 move in circles was true enough to serve the purpose of 

 correlating such observations as were then possible; 

 after Kepler, the assumption that they move in ellipses 

 became true enough in regard to the state of observa- 

 tional astronomy at that time. We say still that the 

 orbits of the planets are ellipses, because, for all ordi- 

 nary purposes, that is a sufficiently near approximation 

 to the truth; but, as a matter of fact, the center of 

 gravity of a planet describes neither an ellipse nor any 

 other simple curve, but an immensely complicated undu- 

 lating line. It may fairly be doubted whether any gen- 

 eralisation, or hypothesis, based upon physical data is 

 absolutely true, in the sense that a mathematical propo- 

 sition is so; but, if its errors can become apparent only 

 outside the limits of practicable observation, it may be 

 just as usefully adopted for one of the symbols of that 

 algebra by which we interpret Nature, as if it were 

 absolutely true. 



The development of every branch of physical knowl- 

 edge presents three stages, which, in their logical relation, 

 are successive. The first is the determination of the sen- 

 sible character and order of the phenomena. This is 

 Natural History, in the original sense of the term, and 



