The tests of scientific expertness are—distrust of the senses; the recognition of the relation of induction to deduction; avoidance of all sources of error, in observation, experiment, or reasoning; the appreciation of the relative importance and non-importance of real or supposed facts; the ability to distinguish reality and its semblance, especially between the subjective and the objective; and, what indeed follows from the other three tests, the maintenance of mental equilibrium during experimental researches. The reputation of an expert is acquired primarily through other experts, and secondarily through the judgment of mankind. Expertness has its various degrees; there are experts, and experts, and experts: the highest experts are the originators, the explorers, the pioneers, the founders, the creators of science; the lowest experts are the followers, the gleaners, the treasurers, the curators of what others have discovered, and who simply repeat and retain the experiments of genius. The highest experts—the Newtons and Galileos, the Harveys and Jenners of science—must stand at first alone, with no other expert at hand, by which to estimate their relative heights and strength; it is their necessity and their glory to educate experts, by whom their own merits are to be tested; they must create the standard by which they are to be judged; their critics will be their own offspring. For many years Newton was the only man on this planet who understood the theory of gravitation, or was able to criticise it.
The tests of scientific non-expertness are, blind repose in the senses, the inability to eliminate or appreciate sources of error in observation, experiment, or reasoning; the non-appreciation of the relative importance of real or supposed facts; the confounding of semblance with reality, and particularly the subjective with the objective; liability to be entranced, or to have the emotions unduly excited, in the presence of genuine or supposed phenomena; and the use of induction when only deduction is valid, and vice versa.
Expertness in one branch of science not only does not qualify, but in various ways may disqualify, one to be an expert in another branch.
That skill in mathematics unfits one for the successful study of various other specialties was pointed out long ago, both by Abercrombie and Hamilton; but the antagonism of specialties may be traced, under recent experience, yet more minutely, for it is found that eminence in physics, or chemistry, or astronomy, may thoroughly unfit one for the study of physiology; and within a few years some of the greatest blunders recorded in the history of delusions have been made by naturalists, chemists, physicists, jurists, and astronomers of unquestioned honesty, real genius, and deserved eminence, making, or attempting to make, discoveries in the new and almost unexplored realm of cerebro-physiology—experiments with living human beings. The worst blunders in the world are scientific blunders—the unconscious slips of justly eminent men, who do not know that their very eminence in one sphere forbids them to undertake another.