Several themes recur in Lovecraft's stories:Now all my tales are based on the fundamental premise that common human laws and interests and emotions have no validity or significance in the vast cosmos-at-large. To me there is nothing but puerility in a tale in which the human form—and the local human passions and conditions and standards—are depicted as native to other worlds or other universes. To achieve the essence of real externality, whether of time or space or dimension, one must forget that such things as organic life, good and evil, love and hate, and all such local attributes of a negligible and temporary race called mankind, have any existence at all. Only the human scenes and characters must have human qualities. These must be handled with unsparing realism, (not catch-penny romanticism) but when we cross the line to the boundless and hideous unknown—the shadow-haunted Outside—we must remember to leave our humanity and terrestrialism at the threshold.
— H. P. Lovecraft, in note to the editor of Weird Tales, on resubmission of "The Call of Cthulhu"
Forbidden, dark, esoterically veiled knowledge is a central theme in many of Lovecraft's works. Many of his characters are driven by curiosity or scientific endeavor, and in many of his stories the knowledge they uncover proves Promethean in nature, either filling the seeker with regret for what they have learned, destroying them psychologically, or completely destroying the person who holds the knowledge.
Some critics argue that this theme is a reflection of Lovecraft's contempt of the world around him, causing him to search inwardly for knowledge and inspiration.
Non-human influences on humanity
The beings of Lovecraft's mythos often have human servants; Cthulhu, for instance, is worshiped under various names by cults among both the Greenlandic Inuit and voodoo circles of Louisiana, and in many other parts of the world.
These worshippers served a useful narrative purpose for Lovecraft. Many beings of the Mythos were too powerful to be defeated by human opponents, and so horrific that direct knowledge of them meant insanity for the victim. When dealing with such beings, Lovecraft needed a way to provide exposition and build tension without bringing the story to a premature end. Human followers gave him a way to reveal information about their "gods" in a diluted form, and also made it possible for his protagonists to win paltry victories. Lovecraft, like his contemporaries, envisioned "savages" as closer to supernatural knowledge unknown to civilized man.
Another recurring theme in Lovecraft's stories is the idea that descendants in a bloodline can never escape the stain of crimes committed by their forebears, at least if the crimes are atrocious enough. Descendants may be very far removed, both in place and in time (and, indeed, in culpability), from the act itself, and yet, they may be haunted by the revenant past, e.g. "The Rats in the Walls," "The Lurking Fear," "Arthur Jermyn," "The Alchemist," The Shadow over Innsmouth, "The Doom that Came to Sarnath" and The Case of Charles Dexter Ward.
Often in Lovecraft's works the protagonist is not in control of his own actions, or finds it impossible to change course. Many of his characters would be free from danger if they simply managed to run away; however, this possibility either never arises or is somehow curtailed by some outside force, such as in "The Colour Out of Space" and "The Dreams in the Witch House." Often his characters are subject to a compulsive influence from powerful malevolent or indifferent beings. As with the inevitability of one's ancestry, eventually even running away, or death itself, provides no safety ("The Thing on the Doorstep," "The Outsider," The Case of Charles Dexter Ward, etc.). In some cases, this doom is manifest in the entirety of humanity, and no escape is possible (The Shadow Out of Time).
Civilization under threat
Lovecraft was familiar with the work of the German conservative-revolutionary theorist Oswald Spengler, whose pessimistic thesis of the decadence of the modern West formed a crucial element in Lovecraft's overall anti-modern worldview. Spenglerian imagery of cyclical decay is present in particular in At the Mountains of Madness. S. T. Joshi, in H. P. Lovecraft: The Decline of the West, places Spengler at the center of his discussion of Lovecraft's political and philosophical ideas.
Lovecraft wrote to Clark Ashton Smith in 1927: "It is my belief, and was so long before Spengler put his seal of scholarly proof on it, that our mechanical and industrial age is one of frank decadence." Lovecraft was also acquainted with the writings of another German philosopher of decadence: Friedrich Nietzsche.
Lovecraft frequently dealt with the idea of civilization struggling against dark, primitive barbarism. In some stories this struggle is at an individual level; many of his protagonists are cultured, highly educated men who are gradually corrupted by some obscure and feared influence.
In such stories, the curse is often a hereditary one, either because of interbreeding with non-humans (e.g., "Facts Concerning the Late Arthur Jermyn and His Family" (1920), The Shadow over Innsmouth (1931)) or through direct magical influence (The Case of Charles Dexter Ward). Physical and mental degradation often come together; this theme of 'tainted blood' may represent concerns relating to Lovecraft's own family history, particularly the death of his father due to what Lovecraft must have suspected to be a syphilitic disorder.
In other tales, an entire society is threatened by barbarism. Sometimes the barbarism comes as an external threat, with a civilized race destroyed in war (e.g., "Polaris"). Sometimes, an isolated pocket of humanity falls into decadence and atavism of its own accord (e.g., "The Lurking Fear"). But most often, such stories involve a civilized culture being gradually undermined by a malevolent underclass influenced by inhuman forces.
It is likely that the "Roaring Twenties" left Lovecraft disillusioned as he was still obscure and struggling with the basic necessities of daily life, combined with seeing non-Western European immigrants in New York City.
Race, ethnicity, and class
Race is the most controversial aspect of Lovecraft's legacy, expressed in many disparaging remarks against the various non-Anglo-Saxon races and cultures in his work. As he grew older, his original Anglo-Saxon racial worldview softened into a classism or elitism which regarded the superior race to include all those self-ennobled through high culture. From the start, Lovecraft did not hold all white people in uniform high regard, but rather esteemed the English people and those of English descent. He praised non-WASP groups such as Hispanics and Jews; however his private writings on groups such as Irish Catholics, German immigrants and African-Americans were consistently negative. In an early poem, the 1912 "On the Creation of Niggers," Lovecraft describes black people not as human but as "beast[s] ... in semi-human figure, filled ... with vice." In his early published essays, private letters and personal utterances, he argued for a strong color line to preserve race and culture. He made these arguments by direct disparagement of various races in his journalism and letters, and perhaps allegorically in his fiction concerning non-human races. Some have interpreted his racial attitude as being more cultural than brutally biological: Lovecraft showed sympathy to those who adopted Western culture, even to the extent of marrying a Jewish woman whom he viewed as "well assimilated." While Lovecraft's racial attitude has been seen as directly influenced by the society of his day, especially the New England society he grew up in, his racism appeared stronger than the general popular viewpoint.
Risks of a scientific era
At the turn of the 20th century, humanity's increased reliance upon science was both opening new worlds and solidifying understanding of ours. Lovecraft portrays this potential for a growing gap of man's understanding of the universe as a potential for horror, most notably in "The Colour Out of Space," where the inability of science to comprehend a contaminated meteorite leads to horror.
In a letter to James F. Morton in 1923, Lovecraft specifically pointed to Einstein's theory on relativity as throwing the world into chaos and making the cosmos a jest; in a letter to Woodburn Harris in 1929, he speculated that technological comforts risk the collapse of science. Indeed, at a time when men viewed science as limitless and powerful, Lovecraft imagined alternative potential and fearful outcomes. In "The Call of Cthulhu," Lovecraft's characters encounter architecture which is "abnormal, non-Euclidean, and loathsomely redolent of spheres and dimensions apart from ours." Non-Euclidean geometry is the mathematical language and background of Einstein's general theory of relativity, and Lovecraft references it repeatedly in exploring alien archaeology.
Lovecraft's works are ruled by several distinct pantheons of deities (actually aliens worshiped as such by humans) who are either indifferent or actively hostile to humanity. Lovecraft's actual philosophy has been termed "cosmic indifference" and this is expressed in his fiction. Several of Lovecraft's stories of the Old Ones (alien beings of the Cthulhu Mythos) propose alternate mythic human origins in contrast to those found in the creation stories of existing religions, expanding on a natural world view. For instance, in Lovecraft's At the Mountains of Madness, it is proposed that humankind was actually created as a slave race by the Old Ones, and that life on Earth as we know it evolved from scientific experiments abandoned by the Elder Things. Protagonist characters in Lovecraft are usually educated men, citing scientific and rational evidence to support their non-faith. Herbert West–Reanimator reflects on the atheism common in academic circles. In "The Silver Key," the character Randolph Carter loses the ability to dream and seeks solace in religion, specifically Congregationalism, but does not find it and ultimately loses faith.
Lovecraft himself adopted the stance of atheism early in life. In 1932, he wrote in a letter to Robert E. Howard:
All I say is that I think it is damned unlikely that anything like a central cosmic will, a spirit world, or an eternal survival of personality exist. They are the most preposterous and unjustified of all the guesses which can be made about the universe, and I am not enough of a hairsplitter to pretend that I don't regard them as arrant and negligible moonshine. In theory, I am an agnostic, but pending the appearance of radical evidence I must be classed, practically and provisionally, as an atheist.
In 1926, famed magician and escapist Harry Houdini asked Lovecraft to ghostwrite a treatise exploring the topic of superstition. Houdini's unexpected death later that year halted the project, but The Cancer of Superstition was partially completed by Lovecraft along with collaborator C. M. Eddy Jr. A previously unknown manuscript of the work was discovered in 2016 in a collection owned by a magic shop. The book states "all superstitious beliefs are relics of a common 'prehistoric ignorance' in humans," and goes on to explore various superstitious beliefs in different cultures and times."