November 19, 1891] 



MATURE 



61 



rest satisfied about any scientific inquiry the truth of 

 which has not been demonstrated, unless we find that it 

 is one which we have no possible power to answer. It 

 would obviously be idle to occupy ourselves about the 

 shape or number of the mountains on that side of the 

 moon which is constantly turned away from us. 



Yet, although doubt and inquiry are necessary in 

 science, nevertheless doubt has its legitimate limits. 

 Blind disbelief is scientifically fatal, as well as blind 

 belief. We all know how apt men are, when seeking to 

 avoid one extreme, to fall into the opposite one, and it is 

 possible to get into an unhealthy condition of mind so as 

 to be unable to give a vigorous assent to anything. It is 

 necessary distinctly to recognize there is such a thing as 

 legitimate certainty, not to perceive the force of which is 

 illegitimate doubt. Such doubt would necessarily dis- 

 credit all physical science. 



Universal doubt, for example, is an absurdity. It is 

 scepticism run mad. 



If anyone affirms that '■'■ 7iothing is ccrtai7t^'' he ob- 

 viously contradicts himself, since he thereby affirms the 

 certainty of uncertainty. He says that, which, if true, 

 absolutely contradicts what he has declared to be true. 



But a man who affirms what the system he professes to 

 adopt forbids him to affirm, and who declares that he be- 

 lieves what he also declares to be unbelievable, should 

 hardly complain if he is called "foolish." No system can be 

 true, and no reasoning can be valid, which inevitably ends 

 in absurdity. Stick scepticism, then, cannot be the mark 

 of an exceptionally intellectual mind, but of an excep- 

 tionally foolish one, and every position which necessarily 

 leads to scepticism of this sort must be an untenable 

 position. 



A very little reflection suffices to show how self-refuting 

 such modes of thought are. 



Thus, if a man were to say, '* / cannot know anything 

 because I cannot be sure that jny faculties are fiot always fal- 

 lacious," or " / can7iot be sure of anything because, for all 

 I know, I may be the plaything of a demon who amuses 

 himself by constantly deceiving me " — in both these cases 

 he contradicts himself, because he obviously grounds his 

 assertion upon his perception of the truth that " we can- 

 not arrive at conchisions which are certain by means of 

 premisses luhich are uncertain or false.'' 



But if he knows that truth, he must know that his 

 faculties are not always fallacious, and that his demon 

 cannot deceive him in everything. 



My object in making these remarks is to enable us to 

 get clear of mere idle, irrational doubts which have no 

 place in science and can have none, so that we may 

 recognize the fact that we all of us have certainty as to 

 some facts according to our degrees of knowledge. Ob- 

 viously we can only judge of truth by our mental faculties, 

 and if a man denies their validity we must pass him by, 

 contenting ourselves with calling his attention to the fact 

 that he refutes himself. If a man professes to doubt his 

 faculties, or to doubt whether language can be trusted to 

 convey thought, then plainly we cannot profitably argue 

 with him. But if, on account of his absurdity, we cannot 

 refute him, it is no less plain that he cannot defend his 

 scepticism. Were he to attempt to do so, then he would 

 show, by that very attempt, that he really had confidence 

 in reason and in language, however he might verbally 

 deny it. 



Confident, then, that there are some scientific state- 

 ments on which we may rely with certainty, let us con- 

 sider a few truths implicitly contained in them. 



In the first place, science makes use not only of obser- 

 vations and experiments, but also of reasoning as to the 

 results of such experiments. It needs that we should 

 draw valid inferences ; but this implies that we may, and 

 must, place confidence in the principle of deduction — in 

 that perception of the mind which we express by the 

 word " therefore." When we use that word, we mean to 



NO. I 151, VOL. 45] 



express by it that there is a truth, the certainty of which 

 is shown through the help of different facts or prin- 

 ciples which themselves are known to be true. 



It is sometimes objected to deductive reasoning— to 

 the syllogism — that it really teaches us nothing new, all 

 that is contained in the conclusion being contained 

 already in the premisses. But this objection is due to a 

 want of perception of the great difference which exists 

 between implicit and explicit knowledge. Let us sup- 

 pose a person to be looking at some very flexible and 

 soft kind of fish. He may, perhaps, say to himself, " This 

 creature can have no spinal column ! " Then it may 

 strike him that naturalists have classed fishes, together 

 with other animals, in a great group, one character of 

 which is the possession of a spinal column, and so he 

 may explicitly recognize a truth implied in what he knew 

 before. So great, indeed, is the difference between explicit 

 and implicit knowledge, that the latter may not really 

 deserve to be called " real knowledge " at all. No one 

 will affirm that a student who has merely learned the 

 axioms and definitions of Euclid has attained such a real 

 knowledge of all the geometrical truths the work contains 

 that he will fully understand all its propositions and 

 theorems without having to study them. Yet all the 

 propositions, &c.j of Euclid are implicitly contained in 

 the definitions and axioms. Nevertheless, the student 

 will have to go through many processes of inference by 

 which these implicit truths may be explicitly recognized 

 by him, before he can be said to have any real knowledge 

 of them. 



The VALIDITY OF INFERENCE is, then, one of the 

 truths implied by physical science, and we shall presently 

 see the intellectual penalty which must be paid for any 

 real doubt about it. 



In the second place, physical science is emphatically 

 experimental science. But every experiment, carefully 

 performed, implies a most important latent truth. For 

 when an experiment has shown us that anything is certain 

 — as, for example, that a newt's leg may grow again, after 

 amputation, because one actually has grown again ; we 

 shall find that such certainty implies a prior truth. It 

 implies the truth that if the newt has come to have four 

 legs once more, it cannot at the very same time have 

 only three legs. This may seem too trivial a remark to 

 some of my hearers, but there is nothing like a concrete 

 example for making an abstract truth plain. Anything 

 we are certain about, because it has been proved to us 

 by experiment, is certain only if we know, and because 

 we know, that a thing which has been actually proved 

 cannot at the same time remain unproven. If we reflect 

 again on this proposition, we shall see that it depends 

 on a still more fundamental truth which our reason 

 recognizes— the truth, namely, that " nothing can at the 

 same time both be and not be" — the truth known as 

 " THE LAW OF CONTRADICTION"; and this I bring forward 

 as a second truth implied by physical science. 



If we reflect upon this law, we shall see that our intellect 

 recognizes it as an absolute and necessary truth which 

 carries with it its own evidence. It is but the summing 

 up in one general expression, of all the concrete separate 

 cases— such as that of the newt's legs, of the fact that if a 

 man possesses two eyes he cannot at the same time have 

 only one, and so on. 



But an objection has been made as follows : " It is 

 very true that I cannot imagine having * two eyes ' and 

 only ' one eye ' at the same time, and so I must practic- 

 ally acquiesce in the statement, but I am only compelled 

 to do so by the impotence of my imagination." Thus, 

 instead of the "law of contradiction," Mr. Herbert Spencer 

 has put forward as an ultimate truth — " his universal 

 postulate" — the assertion that ^^ we must accept as true 

 propositions we cannot help thinking, because we cannot 

 imagine the contrary.'^ But if any of my hearers will 

 reflect over what his mind tells him when it pronounces 



