8 THOUGHTS ON OUR CONCEPTIONS OF PHYSICAL LAW. 



knowledge at second hand are generally more ready with a positive decision and 

 a reason for it, not necessarily because their intellectual integrity is less, but be- 

 cause they cannot realize how vain a thing the human reason is. To imagination 

 and reason, controlled and checked by experiment and observation, are we to 

 look as the source of the greatest advancement in science; but we are not to 

 look for infallibility, and in cases where the reason alone is allowed to decide, 

 where observation and experiment are impossible, the uncertainty must necessa- 

 rily be greater. In many cases the fact that the subject is so intrinsically difficult 

 that no experimental check is possible, appears to inspire the investigator with a 

 confidence in his conclusions, that could hardly be reinforced by absolute cer- 

 tainty. 



But we have not yet exhausted the list of errors to which scientific men are 

 liable, in arriving at what we provisionally call correct conceptions of physical 

 law. A few of them have carried their investigations into a domain in which 

 certain hypothetical beings called *' spirits" are said to be the main actors. It 

 is exceedingly probable that a few trained investigators have been deceived in 

 regard to the evidence of their own senses. This is not an» unlooked for result, 

 as it can be readily reproduced in the performances of any expert juggler. 



I have confined myself to the errors which scientific men have committed, 

 and to which they are liable in their search for truth, not because they alone are 

 liable to err, but because a discussion of the multitude of errors into which intel- 

 lectual men of other professions have fallen, would be sure to give offense. But 

 it is not the scientific mind which stands impeached — it is the human mind ! 



To what end have we then come ? It appears that all scientific results are 

 attended with some uncertainty. Sometimes the uncertainty is very small, and 

 we are able to obtain a numerical estimate of it. In other cases it may be possi- 

 ble that a fundamental misconception of the truth may have been formed. As 

 an instance illustrating what I mean, we may cite the case of the measurement of 

 the Atlanta base-line by the engineers of the United States Coast Survey. The 

 whole length of the base-line was nearly six miles, and three determinations of its 

 length showed differences of about 3-10 of an inch — about a miUionth of the entire 

 length. It is safe to say that if these re-measurements had shown differences of 

 ten feet, there would have been no quarreling in regard to which measurement 

 was right, but all would have been rejected, and if the engineers were not dis- 

 missed as incompetent, they would, with feelings of mortification, have begun 

 their work over again. It is curious to observe that in many cases \yhere less 

 skillful men attack problems infinitely more complex, reaching conclusions differ- 

 ing as widely as the poles, we have, instead of conscientious re-investigation, or a 

 modest reservation of judgment, dogmatic discussions, empty words. 



In the other case, where the error is likely to be a fundamental one, the 

 probability of the truth or error of a conclusion cannot always be determined 

 numerically, and will vary greatly in different minds. Fur instance, A may think 

 he saw a ball dropped into a box, and may feel certain that it is yet there; B is 

 certain that it was a juggler's trick, and that the box is empty ; C did not see 



