Several themes recur in Lovecraft's stories:
Forbidden knowledge
In the opening of his 1926 tale "The Call of Cthulhu" Lovecraft wrote:
The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents. We live on a placid island of ignorance in the midst of black seas of infinity, and it was not meant that we should voyage far. The sciences, each straining in its own direction, have hitherto harmed us little; but some day the piecing together of dissociated knowledge will open up such terrifying vistas of reality, and of our frightful position therein, that we shall either go mad from the revelation or flee from the deadly light into the peace and safety of a new dark age.
Lovecraft's protagonists are nevertheless driven to this "piecing together", which becomes a primary plot device in many of his works.
When such vistas are opened, the mind of the protagonist-investigator is often destroyed. Those who actually encounter "living" manifestations of the incomprehensible are particularly likely to go mad, as is the case of the titular character in
The Music of Erich Zann. The story features an insane mute viol virtuoso's sixth-floor apartment, whose window is the only one high enough to see over a wall on a mysterious, disappearing Parisian street...a wall whose other side contains unexplainable horrors.
Similarly, his last, and arguably most mature novel "At The Mountains Of Madness" reveals the existence of a remote frozen and ancient (non-human) city in the central wastes of Antarctica. Mankind's true origins are revealed (incidental 'playthings' created for amusement by the Great Old Ones). Many of the expedition members are killed, one (Danforth) driven incurably insane by the ancient knowledge and unpleasant events which overtake the expedition. The theme of 'forbidden knowledge' is echoed in the very opening lines of the story by the expedition's leader and survivor (Professor Dyer of the Miskatonic University): "I am forced into speech because men of science have refused to follow my advice without knowing why".
Those characters who attempt to make use of such knowledge are almost invariably doomed. Sometimes their work attracts the attention of malevolent beings; other times, evoking the spirit of Frankenstein, they are destroyed by monsters of their own creation.
Non-human influences on humanity
The beings of Lovecraft's mythos often have human (or mostly human) servants; Cthulhu, for instance, is worshiped under various names by cults amongst both the Eskimos of Greenland and voodoo circles of Louisiana, and in many other parts of the world.
These worshipers 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 the Earth, only in Lovecraft's case, this meant, so to speak, closer to Cthulhu.
Inherited guilt
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, from however remote the past, blood will out ("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").
Fate
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
Though little known to his fan base, Lovecraft was familiar with the work of the German conservative-revolutionary theorist Oswald Spengler. Spengler's pessimistic thesis of the decadence of the modern West formed a crucial element in Lovecraft's overall anti-modern, conservative worldview. Spenglerian imagery of cyclical decay is present in particular in
At the Mountains of Madness. In his book titled
H.P. Lovecraft: The Decline of the West, S. T. Joshi 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" (see China Miéville's introduction to "At the Mountains of Madness", Modern Library Classics, 2005). Lovecraft was also acquainted with the writings of another German intellectual who dealt with civilized decadence in philosophical terms: Friedrich Nietzsche.
Lovecraft frequently dealt with the idea of civilization struggling against more barbaric, primitive elements. 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.
There is a lack of analysis as to whether England's gradual loss of prominence and related conflicts (Boer War, India, World War I) had an impact on Lovecraft's worldview. 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-European immigrants in New York City.
Race, ethnicity, and class
Lovecraft lived at a time when the eugenics movement, anti-Catholicism, Antisemitism, nativism, and strict racial segregation and miscegenation laws were all widespread in the United States, and his writings reflect that social and intellectual environment. A common dramatic device in Lovecraft's work is to associate virtue, intellect, civilization, and rationality with upper class White Anglo-Saxon Protestants. These are often posed in contrast to the corrupt, intellectually inferior, uncivilized and irrational attributes which he associated with both the lower classes in general and those of non-Anglo Saxon ethnicity, especially those who have dark skin. He held English culture to be the comparative pinnacle of civilization, with the descendants of the English in America as something of a second-class offshoot, and everyone else below.
Ethnicity was more salient than race for Lovecraft; he admired Anglo-Saxons in particular, not white people generally. Non-Anglo-Saxon whites of European descent are frequently disparaged in his work on ethnic grounds. The degenerate descendants of Dutch immigrants in the Catskill Mountains, "who correspond exactly to the decadent element of white trash in the South", are common targets. In "The Temple", Lovecraft's highly unsympathetic narrator is a German World War I U-boat captain whose faith in his "iron German will" and the superiority of the Fatherland lead him to machine-gun helpless survivors in lifeboats and, later, kill his own crew, while blinding him to the curse he has brought upon himself.
Class distinctions inform Lovecraft's worldview nearly as much as ethnicity. The narrator of "Cool Air" speaks disparagingly of the poor Hispanics of his neighborhood, but respects and admires the wealthy and aristocratic Dr. Muñoz, described as "a man of birth, cultivation, and discrimination."
S. T. Joshi notes, "There is no denying the reality of Lovecraft's racism, nor can it merely be passed off as 'typical of his time', for it appears that Lovecraft expressed his views more pronouncedly (although usually not for publication) than many others of his era. It is also foolish to deny that racism enters into his fiction." In his book
Against the World, Against Life, Michel Houellebecq argues that "racial hatred" provided the emotional force and inspiration for much of Lovecraft's greatest works.
According to L. Sprague de Camp's biography, Lovecraft greatly moderated his views toward the end of his life as he began to travel more and came into contact with people of diverse ethnic backgrounds. He says Lovecraft was horrified by reports of anti-Jewish violence in Germany during the 1930s, which he regarded as irrational. Sprague de Camp also says that Lovecraft enjoyed getting a rise out of people he considered his intellectual inferiors by stating in a deadpan manner whatever he thought would offend them the most, and suggests that at least some reports of Lovecraft's racism derived from this practice.
Risks of a scientific era
At the turn of the 20th century, man's increased reliance upon science was both opening new worlds and solidifying the manners by which he could understand them. 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", the inability of science to comprehend a contaminated meteorite leads to horror.
In a letter to James F. Morton in 1923, Lovecraft specifically points to Einstein's theory on relativity as throwing the world into chaos and making the cosmos a jest. And in a 1929 letter to Woodburn Harris, he speculates 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-Euclidian, 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 will reference it again and again in exploring alien archeology.
Religion
Misotheism is a recurrent theme in Lovecraft fiction. Many of Lovecraft's works are directly or indirectly adversarial to the belief in a loving, protective God; Lovecraft's works are ruled by several distinct pantheons of deities who are either indifferent or actively hostile to humanity. Several, particularly those of the Cthulhu Mythos, indulge upon alternate human origins in contrast to those found in Genesis and creation stories of other religions. Protagonist characters are often educated men who favor the claims of the physical sciences over those of scripture.
Herbert West—Reanimator, reflects on the atheism common within academic circles. Also in Through the Gates of the Silver Key the character Randolph Carter attempts after losing access to dreams to seek solace in religion, specifically Congregationalism, but doesn't and ultimately loses faith.
Lovecraft himself adopted the stance of atheism early in his 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 hair-splitter 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. "