When people talk about the PI, they always trace the character back to three writers: Dashiell Hammett, Raymond Chandler, and Ross MacDonald. Most people think the modern PI is based on Hammett's Continental Op. But you have to go farther back than that. Sherlock Holmes?
Well, yes, Holmes's fingerprints are all over the modern PI. He even has an erudite, if seemingly less intelligent, sidekick, the brainier forerunner of the psycho sidekick popularized in the Spenser and Dennis Lehane novels. But you have to go farther back. And I mean farther back than Poe's August Dupin, considered the first modern detective character.
No, the PI is a knight-errant, righting wrongs, defending the weak, and dispensing justice. The knight-errant was around for centuries, springing from stories of Siegfried the dragon-slayer, of Roland and Charlemagne, of the various knights of Camelot. But the archetype wasn't truly solidified until Miguel de Cervantes's comic novel, The Ingenious Gentleman Don Quixote of La Mancha (which I plan to review this Friday.)
[Cue needle across vinyl.] "Duh... What? Don Quixote was nuts! And his sidekick was equal parts wise and ignorant."
The comic aspect of that dynamic did not really repeat on a grand scale until the classic 1980's sitcom, Blackadder. In the beginning, Prince Edmund, the Black Adder, is more bungling than mad, and sidekick Baldric is much smarter than he appears, frequently saving the hapless prince from himself. Later, the roles were flipped, and Blackadder became an evil version of Holmes - arrogant, clever, and just as sarcastic - while Baldric became the embarrassingly dimwitted sidekick who always had "a cunning plan" (that always ended in disaster.)
So what's this have to do with the PI?
Think about Holmes, particularly as portrayed at present by Messrs. Downey, Cummerbatch, and Miller. The modern depictions of Holmes have more in common with Blackadder than in prior decades, while Watson is portrayed as long-suffering and sometimes the source of Holmes' brilliance. This was Doyle's original vision of the pair, and you can draw a direct line back to Don Quixote and Sancho Panza. But whereas the don was off his rocker and Sancho had a simple view of the world, the impulses were the same: Quixote, like Holmes and like every PI character who followed him, loathes injustice and wants to see things set right. Panza, like Watson and the later stock psycho sidekicks, sees Quixote's (or Holmes's or Spenser's or Patrick and Angie's) mission as noble, though often has to show great patience standing in his brilliant partner's shadow.
The motivations and the levels of intelligence change. Even the personal missions change. Spenser, never mind Holmes, could not have thrived in the time of King Arthur or Charlemagne. And the whole thrust of Don Quixote's story is that the knight-errant was already part of a fictional past.
The PI is not the only evolution of Don Quixote, but it's the most obvious. Fans of Doctor Who can pick up on Quixote's madness in the Doctor, but it's darker and more bizarre. And more intentional.
So Don Quixote is still alive. When the PI is done right, the character taps into that zeitgeist. When it's not, he or she is simply parroting the Op and Marlowe.