There is no widely agreed upon definition of the Great Detective but mine would go like this:
An amazingly gifted person, either from an amateur or professional background, who solves criminal cases that leave other people baffled.
While it may seem rather broad and consensual this definition is much more restrictive than appears at first sight, for there are many fictional detectives, some of them quite popular, that don’t match it. It’s especially true of the « Beta Detectives » of post-war and modern crime fiction, very few of whom can be called « amazingly gifted » in the sense Sherlock Holmes or Hercule Poirot were. They sure solve their cases but painstaking work and chance, not little grey cells, are their best allies – and often the criminal is obliging enough to confess in detail, sparing the detective the trouble of filling the blanks. As said above, this lack of sleuthing prowess doesn’t keep those detectives from enjoying a big, sometimes massive, following thanks to their colourful personalities and/or eventful private lives. To many, Alan Banks is every bit as much of a Great Detective as, say, Gideon Fell (assuming they have ever heard of the latter) but is that true? I’m not so sure, and not just because of my personal preferences.
When assessing the « Greatness » of a fictional detective it is important I think to distinguish between their popularity and their actual abilities. Whether a literary sleuth is or is not popular with readers has no bearing on their « Great Detective » status. I can think of many a « Great Detective » according to my definition that are or no longer are familiar to the average reader and just as many « Beta Detectives » that enjoy best-seller status – and vice and versa. We should also distinguish between the person and the detective: A great character is not necessarily a great sleuth. Ultimately the Big Question is, why do we follow this detective’s peregrinations? Depending on the answer, you have a Great Detective or a « merely » Famous one.
Don’t get me wrong: I have not turned overnight into an elitist prig. I’m definitely not trying to insinuate that Famous Detectives are a lesser article, for they are not. Making their protagonist an Alpha or Beta sleuth is a writer’s indisputable prerogative and the ultimate criterion is whether it works or not in the context of the story. Also the lines are sometimes more blurry than it seems: Where do we put Maigret on the map? He is in turn an Alpha or a Beta detective and he is indisputably popular. Still, I think – but it’s just me – that words have a meaning and that a Great Detective must be one that is Great at Detecting, which isn’t as frequent or self-evident as it may seem.