52 THE METHOD OF SCIENCE 



results. 1 Mathematics, the most exact of all sciences, is almost 

 purely deductive. Many of the more important biological 

 generalizations could not have been confirmed or even reached 

 without the employment of deduction. 2 Deduction is as essential 

 a part of human mental operations as induction. We seldom 

 use the word ' therefore ' except when our thinking is deductive. 

 All calculation is deduction. Our every-day thinking that 

 incessant play of thought, that associating, disassociating, 

 comparing, discriminating, inferring, in which we are incessantly 

 engaged is largely, indeed mainly, deductive. We gather facts 

 from the world around us and by processes of induction reach 

 thousands or tens of thousands of conclusions. But, like the parts 

 of a scattered machine, the universe our mind constructs for us 

 would have no meaning, no unity, and our knowledge little 

 utility, unless these inductions were linked together, and amplified, 

 and multiplied a thousand-fold by deduction. It is hardly possible, 

 even for a young child, to become aware of a sight, sound, odour, 

 taste, or tactile sensation without, consciously or unconsciously, 

 recalling facts and generalizations previously formulated, and 

 linking the new experience to them by deductive thinking. Nature 

 drives us ceaselessly to the acquirement of skill in deduction, 

 without which we would be lost in a chaos. Our intelligence is due 

 to it. The scientific imagination is usually but another name for 

 its daring yet restrained and careful employment. Not a single 

 monument of the power of human thought, embodied in the 

 literature of the past, but is full of it. No great man of action or 

 thought, from statesman and general to traveller and scientific 

 worker, has existed but was a master of it. Not even the simplest 

 tool was ever thought of, made, or used but through the employ- 

 ment of it ; indeed the whole history of invention and the application 

 of invention is but a chapter in the history of deductive thought. 

 Only by means of it, by appealing from our inductions back again 

 to the facts, can we, especially when some complex matter is under 

 consideration, take the whole of the evidence into account. As 

 a mode of thinking it is an adaptation, the highest product of 



1 " Newton's comprehension of logical method was perfect ; no hypothesis 

 was entertained unless it was definite in conditions, and admitted of unquestion- 

 able deductive reasoning, and the value of each hypothesis was entirely decided 

 by the comparison of its consequences with facts." (Jevons, Principles of Science, 



P- 583.) 



2 For example the theories of evolution and natural selection, both of which, 

 moreover, were founded on facts that had been collected mainly by simple 

 observation. See 350-1. 



