Having retired, I now have the leisure to write my long-promised magnum opus, in which I pull together into a manual the various principles I have derived during a long career as a consulting detective.

First, let me make clear that those principles do not apply only to the solution of crimes. They are the general principles of scientific method, which apply to all research.¹ My little exercises, as documented by my friend and colleague, Dr. John H. Watson, are merely illustrations of those principles in action, within a domain in which they can be concretely demonstrated. The various sciences differ only superficially in their technologies but not in their basic principles, as expounded here. Observation is observation, regardless of what is being observed. If what you must observe is too far away to observe clearly, then you need a telescope to bring it close enough, if what you must observe is too small, then you need a microscope to make it large enough. ² The principles of observation (see Chapter 1), despite those superficial differences, apply equally well to the work of detectives, astronomers, and biologists. Deduction (see Chapter 2) puts content into context, regardless of the type of content.

However, since all you know of my work is that corpus of 60 cases, which Watson published in a forty-year period between 1887 and 1927, I will use them to illustrate the principles. It is important to keep in mind that they apply to those larger issues in which the investigator asks questions of nature. From time to time, I will make this point clear. You may consider that, by doing so, I stray from my area of expertise, but I assure you that Watson captured only a limited aspect of my investigations.

Since my revelations here would be somewhat embarrassing to my good friend, I will not publish them until his death. In case, perchance, by some accident, I precede him in that final journey, I will hide this manuscript. There is no one else to whom I can entrust it. Watson attested to my solitary nature, and no one has replaced him. It should take some time to find it. A detective is essentially a searcher and any child who has played hide-and-seek knows that the good seekers are also the good hiders. However, anyone who has a deep interest in my work, as described by Watson, has sufficient clues in that corpus to find it. Should there be no such person, then it will indeed never be exposed to the light of public scrutiny. This is entirely appropriate, since it will be obvious that my work has not generated enough interest to justify this further imposition on the valuable time of a future generation of readers.

2   This principle can be extended to a tool not available to Holmes (though it is fascinating to speculate what he would have done with it). If what you must observe is too complex to observe clearly, then you need a computer to make it simple enough.