H.P. Lovecraft was much more than a brilliant and innovative writer and way much more than an unstoppable letter-writing machine. He was an insightful literary critic too, a talent helped by the breadth of his knowledge and his deeply misanthropic outlook. Don't be mistaken though; criticism is something far more important than simply deriding or suggesting a text. When it's done well, it aims to shape a medium or a movement, and that is the reason why HPL's Supernatural Horror in Literature is an excellent and definitive piece on horror.
The oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown. These facts few psychologists will dispute, and their admitted truth must establish for all time the genuineness and dignity of the weirdly horrible tale as a literary form. Against it are discharged all the shafts of a materialistic sophistication which clings to frequently felt emotions and external events, and of a naively insipid idealism which deprecates the aesthetic motive and calls for a didactic literature to uplift the reader toward a suitable degree of smirking optimism. But in spite of all this opposition the weird tale has survived, developed, and attained remarkable heights of perfection; founded as it is on a profound and elementary principle whose appeal, if not always universal, must necessarily be poignant and permanent to minds of the requisite sensitiveness.