"The Delta Factor," first published in January 1975 in the Southern Review, sets out the overall themes of the entire book. Percy begins by asking why modern man is so sad despite the 20th century's technological innovations and unprecedented levels of comfort. More specifically, he is interested in why man feels happy in bad situations and sad in good situations (a question also posed in his novel The Last Gentleman). He posits that this overarching sadness is due to contemporary society's position between two ages: the modern age, which is more or less slowly becoming out of date, and a new age, which is dawning but has not yet truly dawned. The anthropological theories of the modern age, according to Percy, "no longer work and the theories of the new age are not yet known" (7). Percy therefore sees his task as coming up with a new theory of man, which he chooses to center on language, man's attribute that separates him from the animals; The Message in the Bottle will attempt to explain man's strange behavior and unexplained sadness by explaining how man deals with language and symbols.
Percy says that the current theories of man make him into a sort of monster, a "centaur organism-plus soul . . . one not different from beasts yet somehow nevertheless possessing 'freedom' and 'dignity' and 'individuality' and 'mind' and such" (9). Modern man is, then, the collision of Judeo-Christian ethics and its focus upon individual freedom and scientific behaviorism, which says that man is no different from the animals—in other words, modern man believes himself to be no different from animals and yet somehow above them. What's more, no existing research really deals with the question of how language really works, of how human beings use and understand the symbols of linguistics. Percy puts this question into a sort of no-man's land, what he calls a "terra incognita" (17), between linguistics and psychology, the former of which deals with the results of language and the latter of which deals with the way people respond to language.
The Delta Factor, Percy's theory of language, is framed in the context of the story of Helen Keller's learning to say and sign the word water while Annie Sullivan poured water over her hands and repeatedly made the signs for the word into her hand. A behaviorist linguistic reading of this scene might suggest a causal relationship—in other words, Keller felt Sullivan's sign-language stimulus in her hand and in response made a connection in her brain between the signifier and the signified. This is too simplistic a reading, says Percy, because Keller was receiving from both the signifier (the sign for water) and the referent (the water itself). This creates a triangle between water (the word), water (the liquid), and Helen, in which all three corners lead to the other two corners and which Percy says is "absolutely irreducible" (40). This linguistic triangle is thus the building block for all of human intelligence. The moment when this Delta Δ entered the mind of man—whether this happened via random chance or through the intervention of a deity—he became man.
Further, in Delta Δ, the corners of the triangle are removed from their behaviorist contexts. Helen Keller, in other words, becomes something other than just an organism in her environment because she is coupling two unrelated things--water the word and water the liquid—together. Likewise, water the liquid is made something more than water the liquid because Keller has coupled it with the arbitrary sound water, and water the word becomes more than just the sound of the word water (and the shape of the sign language for water). In this way, "the Delta phenomenon yielded a new world and maybe a new way of getting at it. It was not the world of organisms and environments but just as real and twice as human" (44)--man is made whole by the Delta Δ where the popular notions of religion and science had split him in two.