The obscure problems of psychology had a great attraction for him, and he spent much time in investigating them, being one of the founders, and remaining all through his later life a leading and guiding member, of the Society for Psychical Research, which has for the last twenty years cultivated this field with an industry and ability which have deserved larger harvests than have yet been reaped. Two remarkable men, both devoted friends of his, worked with him, Edmund Gurney and Frederic Myers the poet, the latter of whom survived him a few months only. It was characteristic of Sidgwick that he never committed himself to any of the bold and possibly over-sanguine anticipations formed by some of the other members of the Society, while yet he never was deterred by failure, or by the discovery of deceptions, sometimes elaborate and long sustained, from pursuing inquiries which seemed to him to have an ultimate promise of valuable results. The phenomena, he would say, may be true or false; anyhow they deserve investigation. The mere fact that so many persons believe them to be genuine is a problem fit to be investigated. If they are false, it will be a service to have proved them so. If they contain some truth, it is truth of a kind so absolutely new as to be worth much effort and long effort to reach it. In any case, science ought to take the subject out of the hands of charlatans.
The main business of his life, however, was teaching and writing. Three books stand out as those by which he will be best remembered--his _Methods of Ethics_, his _Principles of Political Economy_, and his _Elements of Politics_. All three have won the admiration of those who are experts in the subjects to which they respectively relate, and they continue to be widely read in universities both in Britain and in America. All three bear alike the peculiar impress of his mind.
It was a mind of singular subtlety, fertility, and ingenuity, which applied to every topic an extremely minute and patient analysis. Never satisfied with the obvious view of a question, it seemed unable to acquiesce in any broad and sweeping statement. It discovered objections to every accepted doctrine, exceptions to every rule. It perceived minute distinctions and qualifications which had escaped the notice of previous writers. These qualities made Sidgwick's books somewhat difficult reading for a beginner, who was apt to ask what, after all, was the conclusion to which he had been led by an author who showed him the subject in various lights, and added not a few minor propositions to that which had seemed to be the governing one. But the student who had already some knowledge of the topic, who, though he apprehended its main principles, had not followed them out in detail or perceived the difficulties in applying them, gained immensely by having so many fresh points presented to him, so many fallacies lurking in currently accepted notions detected, so many conditions indicated which might qualify the amplitude of a general proposition. The method of discussion was stimulating. Sometimes it reminded one of the Socratic method as it appears in Plato, but more frequently it was the method of Aristotle, who discusses a subject first from one side, then from another, throws out a number of remarks, not always reconcilable, but always suggestive, regarding it, and finally arrives at a view which he delivers as being probably the best, but one which must be taken subject to the remarks previously made. The reader often feels in Sidgwick's treatment of a subject as he often feels in Aristotle's, that he would like to be left with something more definite and positive, something that can be easily delivered to learners as an established truth. He desires a bolder and broader sweep of the brush. But he also feels how much he is benefited by the process of sifting and analysing to which every conception or dogma is subjected, and he perceives that he is more able to handle it afterwards in his own way when his attention has been called to all these distinctions and qualifications or antinomies which would have escaped any vision less keen than his author's. For those who, in an age prone to hasty reading and careless thinking, are disposed to underrate the difficulties of economic and political questions, and to walk in a vain conceit of knowledge because they have picked up some large generalisations, no better discipline can be prescribed than to follow patiently such a treatment as Sidgwick gives; nor can any reader fail to profit from the candour and the love of truth which illumine his discussion of a subject.