A large portion of what Mencken wrote was criticism, particularly of books. He was in fact probably America's most prominent book critic of the 20th century. This caused many to wonder why he chose to be a critic of other men's ideas rather than an expositor of his own, as well as to wonder what ideas he truly believed in himself. He was also a figure of such controversy that many other learned men took to speculate on what his real motivation was for writing. In a 1923 article, he answered these questions with his usual amusement and honesty.
Ask a professional critic to write about himself and you simply ask him to do what he does every day in the practice of his art and mystery. There is, indeed, no criticism that is not a confidence, and no confidence that is not self-revelation. When I denounce a book with mocking and contumely, and fall upon the poor author in the brutal, Asiatic manner of a drunken longshoreman, a Ku Kluxer, or a midshipman at Annapolis, I am only saying, in the trade cant, that the fellow disgusts me - that his ideas and his manner are somehow obnoxious to me, as those of a Methodist, a golf player or a clog dancer are obnoxious to me - in brief, that I hold myself to be a great deal better than he is and am eager to say so. And when, on the other hand, I praise a book in high, astounding terms, and speak of the author as if his life and sufferings were of capital importance to the world, then I am merely saying that I detect something in him, of prejudice, tradition, habit of mind, that is much like something in myself, and that my own life and sufferings are of the utmost importance to me. That is all there ever is in criticism, once it gets beyond cataloguing. No matter how artfully a critic may try to be impersonal and scientific, he is bound to give himself away.
With criticism thus so transparent, so unescapably revelatory, I often marvel that the gentlemen who concern themselves with my books, often very indignantly, do not penetrate more competently to my essence. Even for a critic I am excessively garrulous and confidential; nevertheless, it is rare for me to encounter a criticism that hits me where I live and have my being. A great deal of ink is wasted trying to discover and denounce my motive in being a critic at all. I am, by one theory, a German spy told off to flay, terrorize and stampede the Anglo-Saxon. By another I am a secret radical, professing to admire Coolidge and Genghis Khan. By a third, I am a fanatical American chauvinist, bent upon destroying and ruining the motherland. All these notions are nonsense; only the first one has even the slightest plausibility. The plain truth is - and how could it be any plainer? - that I practice criticism for precisely the same reason that every other critic practices it: because I am a vain fellow, and have a great many ideas on all sorts of subjects, and like to put them into words and harass the human race with them. If I could confine this flow of ideas to one subject I'd be a professor and get some respect. If I could reduce it, say, to one idea a year, I'd be a novelist, a dramatist, or a newspaper editorial writer. But being unable to stanch the flux, and having, as I say, a vast and exigent vanity, I am a critic of books, and through books of Homo sapiens, and through Homo sapiens of God.
So much for the motive. What, now, of the substance? What is the fundamental faith beneath all the spurting and coruscating the ideas that I have just mentioned? What do I primarily believe in, as a Puritan believes in Hell? I believe in liberty. And when I say liberty, I mean the thing in its widest imaginable sense - liberty up to the extreme limits of the feasible and the tolerable. I am against forbidding anybody to do anything, or say anything, or think anything, so long as it is at all possible to imagine a habitable world in which he would be free to do, say and think it. The burden of proof, as I see it, is always upon the lawmaker, the theologian, the right-thinker. He must prove his case doubly, triply, quadruply, and then he must start all over and prove it again. The eye through which I view him is watery and jaundiced. I do not pretend to be "just" to him - any more than a Christian pretends to be just to the Devil. He is the enemy of everything I admire and respect in this world - of everything that makes it various and amusing and charming. He impedes every honest search for the truth. He stands against every sort of good will and common decency. His ideal is that of an animal trainer, an archbishop, a major-general in the Army. I am against him until the last galoot's ashore.
This simple and childlike faith in the freedom and dignity of man - here, perhaps, stated with undue rhetoric - should be obvious, I should think, to every critic above the mental backwardness of a Federal judge. Nevertheless, very few of them, anatomizing my books, have ever showed any sign of detecting it...
For liberty, when one ascends to the levels where ideas swish by and men pursue Truth to grab her by the tail, is the first thing and the last thing. So long as it prevails the show is thrilling and stupendous; the moment it fails the show is a dull and dirty farce.