PREFACE. 



The distinction between theory and practice is one that is generally recognised in all 

 departments of human affairs. By theory in this context is meant pure knowledge— 

 the knowledge of things as they are apart from any use which may be made of the 

 knowledge. This kind of knowledge— the result of the most careful investigation, 

 continually corrected by improved experiments and more widely extended observation, 

 and subjected to the most rigid criticism by successive generations of enquirers— is 

 pure science. Practice, on the other hand, or practical knowledge, is the knowledge 

 of the methods by which the material and the forces of nature can be made to 

 satisfy human needs and desires. Practice necessarily depends on some knowledge 

 of the properties and relations of natural objects and forces, though in many cases 

 it may simply consist in knowing that a certain desired result will generally be 

 produced by particular operations. This is the kind of knowledge possessed by 

 men trained and experienced in particular " trades " or crafts. 



The distinction has existed since the very commencement ' of human civilisation. 

 The earliest representatives of humanity, like the most primitive savages now existing, 

 had some knowledge of their surroundings which was not directly useful to them, while 

 they understood very little the arts which they practised. To some extent the develop- 

 ment of pure science and that of practical science have proceeded independently, but to 

 a larwe extent they have influenced one another. Practical science has often received a. 

 great impetus from the discoveries of abstract inquiry, and pure science has often mads 

 enormous strides by the study of the results exhibited by industrial processes. On the 

 whole the tendency of the development of the two is towards a perfect harmony in 

 which the knowledge of the universal interaction of natural forces would completely 



