KINDS OF THINKING 355 



we test and establish (or disprove) by making a deductive appeal 

 to reality. In other words we reason. The kind of thinking 

 employed in the one case differs sharply from that employed in 

 the other. For example, if a systematic botanist wishes to 

 demonstrate the truth of a disputed statement, his main task, as 

 a rule, is to prove the correctness of the data from which he 

 starts his thinking. In other words, he has to demonstrate that 

 he has observed, recollected, and compared correctly ; for, while 

 his facts are often multitudinous and complex, the thinking 

 founded on them is usually relatively simple, and therefore 

 unlikely to be wrong if the facts are correct. The mathematician, 

 on the contrary, usually founds thinking that is relatively diffi- 

 cult on facts that are comparatively simple. His main task is to 

 prove the correctness of his reasoning. We shall see later that 

 skill in thinking, whether of the kind employed by the systematist 

 or that which is used by the mathematician, is an ' acquirement.' 

 It develops under the stimulus of use. Moreover, skill in the one 

 kind of thinking does not necessarily imply skill in the other kind. 

 No one disputes that we have created true science when we have 

 arranged our facts * systematically.' Nor has any one ventured to 

 deny that mathematics and physics, in which causal relations are 

 traced, are products of real scientific thought. Occasionally, 

 however, a biologist, while neither denying the facts nor contro- 

 verting the thinking founded on them, denounces ' speculation ' 

 and ' theorizing.' 1 It is unscientific, of course, to found speculations 

 on imaginary facts or formulate guesses which are incapable of 

 proof or disproof; and, certainly, hypotheses of this kind are not 

 uncommon in biology. But, judging from the language sometimes 

 used, it is not merely the faulty use of a right method of work 

 that is condemned, but the method itself. Manifestly, however, if 

 it be possible to link facts in chains of causation, it is proper to do 

 so. Knowledge is then supplemented by more knowledge, and by 

 understanding besides, and the structure of science is all the more 

 complete because we have added the woof to the warp of it. 

 Since verified facts are abundant in biology, and since as living 

 beings we are familiar with the conditions of life, it should not be 

 beyond the powers of the human intellect, which has discovered 

 the calculus and the law of gravitation, to indicate in general 

 terms the uniformities of sequence under which the structures of 

 plants and animals have arisen. At any rate the attempt to do so 

 is not unscientific. 2 



1 See 826. * See 828, et seq. 



