PREFACE TO THE FIRST EDITION. ix 



learnt to advocate the use of hypotheses. I take the 

 extreme view of holding that Francis Bacon, although he 

 correctly insisted upon constant reference to experience, 

 had no correct notions as to the logical method by which 

 from particular facts we educe laws of nature. I endea- 

 vour to show that hypothetical anticipation of nature is 

 an essential part of inductive inquiry, and that it is the 

 >yewtonian method of deductive reasoning combined with 

 elaborate experimental verification, which has led to all 

 the great triumphs of scientific research. 



In attempting to give an explanation of this view of 

 Scientific Method, I have first to show that the sciences 

 of number and quantity repose upon and spring from the 

 simpler and more general science of Logic. The Theory 

 of Probal)ility, which enables us to estimate and calculate 

 quantities of knowledge, is then descril)ed, and especial 

 attention is drawn to the Inverse Method of Probabilities, 

 which involves, as I conceive, the true principle of in- 

 ductive procedure. No iaductive conclusions are more 

 than ]>robable, and I adopt the opinion that the theory of 

 probability is an essential part of logical method, so that 

 the logical value of every inductive result must be deter- 

 mined consciously or unconsciously, according to the 

 principles of the inverse method of probability. 



The phenomena of nature are commonly manifested 

 in quantities of time, space, force, energy, &c., and the 

 observation, measurement,- and analysis of the ^various 

 quantitative conditions or results involved, even in a 

 simple experiment, demand much employment of system- 

 atic procedure. I devote a book, therefore, to a simple 

 and general description of the devices by which exact 

 measurement is effected, errors eliminated, a probable 

 mean result attained, and the probable error of that mean 

 ascertained. I then proceed to the principal, and probably 

 the most interesting, subject of the book, illustrating 

 successively the conditions and precautions requisite fur 



