The best preparation may be relaxation

A "related" discipline - whatever your domain - is psychology. Every discipline involves people and thus an understanding of people is an aid in all disciplines, Indeed, the whole art of deduction could be considered as applied psychology. Although I have had no formal training in psychology, I have taught myself many of its principles.

It was this knowledge of psychology which alerted me to the need to be prepared but not to over-prepare. After working diligently on a problem in one's conscious mind, there comes a point at which it is necessary to relax and allow the unconscious mind to take over. Watson has referred to my capacity to relax - to spend a leisurely afternoon engrossed in listening to music [REDH] and to become immersed in some - to Watson - abstruse topic:

One of the most remarkable characteristics of Sherlock Holmes was his power of throwing his brain out of action and switching all his thoughts on to lighter things whenever he had convinced himself that he could no longer work to advantage. I remember that during the whole of that memorable day he lost himself in a monograph which he had undertaken upon the Polyphonic Motets of Lassus [BRUC].
Serendipity plays an important role in the history of detection, within crime and science in general. That is, some trivial environmental event triggers a solution which suddenly emerges from the unconscious mind. This usually happens when the person is relaxing. Archimedes in his bath-tub, when the overflow of water produces the principle of specific gravity; Newton under an apple tree, when a falling apple triggered the universal theory of gravity. Serendipity, however, strikes only the prepared mind. Both Archimedes and Newton had been working on their respective problems for some time and were therefore sensitized to their solutions. There is no point in sitting in bath-tubs and under apple trees waiting for inspiration without having shed the prerequisite perspiration.

When I found a discharged servant with a grievance, I said: I call it luck, but it would not have come my way had I not been looking for it [WIST]. I got a new hypothesis of the location of Jonathan Small only after, as I explained to Watson, I gave my mind a thorough rest by plunging into a chemical analysis. One of our greatest statesmen has said that a change of work is the best rest [SIGN].

Some of my hobbies had apparently nothing to do with detective skills - they are avocations rather than vocations. However, they do serve a function. They enable me to relax and, in this relaxation, incubation occurs.

Music obviously serves this function. It would stretch the imagination to see how the Polyphonic Motets of Lassus could contribute to the solution of a crime.

--- he lost himself in a monograph which he had undertaken upon the Polyphonic Motets of Lassus [BRUC].
Chemistry would appear to relate to my career. Indeed, when Watson first met me, I was excited about the discovery of a chemical which reacted only to bloodstains. Yet there is no point at which this is used throughout our subsequent forty years of collaboration.

At a deeper level, however, they are all related. Music (note that I study polyphonic rather than homophonic forms of vocal compositions) and chemistry deal with complex structures which provide practice in dealing with complexity.