Dreams
Inception begins—and perhaps ends—inside of a dream, and every scene asks the viewer to assess whether what they are seeing is in fact a dream or not. In the film, dreams are imagined arenas where one's secrets, traumas, and unresolved feelings are vulnerable to exposure and extraction. Cobb and Arthur are first seen in Saito's dream trying to extract business secrets for a company called Cobol Corporation, and later must infiltrate Robert Fischer's dreams. As Cobb explains to Ariadne, dreams are populated by "projections" of the dreamer's mind, and Cobb's mind in particular is haunted by feelings of guilt over the memory of his dead wife Mal. Cobb warns Ariadne never to build dream-worlds by using memories, which can be too seductive and lure the dreamer away from reality.
Reality
Christopher Nolan stated that he wanted Inception to reflect the idea that dreams seem real for as long as they are happening. Reality, or the veneer of reality, is a key aspect to Inception, one that informs not only the scenes where the characters inhabit the external world but also the ones where the characters are dreaming. In order for a constructed dream to be effective, it must essentially pose as reality, which is why Nash's subtle mistake in reconstructing Saito's carpeting compromises the mission in the film's opening sequence. Nolan constantly blurs the distinction between reality and dreaming: the film opens in a dream-within-a-dream that the viewer could easily mistake for reality, and Mal kills herself because she thinks reality is yet another dream.
Warfare
Inception is a film that revolves around the use of military-grade technology in a rivalry between two multinational corporations, led by Saito and Robert Fischer. Although in the external world this rivalry takes the form of corporate espionage, in the dream-world this hostility often degenerates into outright violence and warfare. According to the science-fiction premise of the film, in which "dream-sharing" technology has rendered thoughts vulnerable to theft, high-value targets like Saito and Fischer must militarize their subconscious against unwanted threats. Nolan uses the theme of warfare to show how the mind naturally protects itself against manipulation and coercion, such as when the projections in Fischer's dream launch a coordinated ambush to ward off Cobb's team's infiltration attempt.
Memory
Cobb can no longer build dreams because he still harbors guilt over the circumstances of his wife's death, and is thus haunted by the projection of her memory. Although Cobb initially tells Ariadne she should never use memories to build dreams, Ariadne later discovers Cobb sharing intimate moments with his memories of his wife (for example, in their anniversary suite) in the recesses of his mind. In Inception, memory is a theme that reflects how unresolved attachments one has to the past can linger in the subconscious, foreclosing the possibility of true happiness. Mal, in particular, is a character that embodies Cobb's toxic and self-destructive tendency to live in a theater of memory where she is still alive. Ariadne eventually pushes Cobb to detach himself from the memory of Mal.
Architecture
In Inception, the creation of dream worlds requires a skilled architect, who can painstakingly craft the surfaces and structures of the world itself. In dreams, fluid, shifting, and paradoxical architectural forms are all possible, such as the Penrose staircase that Arthur shows to Ariadne. Nolan uses architectural complexity as a way to reflect the complexity of the human mind. For example, Cobb first orders Ariadne to draw pen-and-paper mazes before showing her how to construct dream-worlds, as a way to introduce her to the labyrinthine environments that the team will need in order to avoid projections. The surreal architecture of dream-spaces, like the hidden operating room behind the control panel of Fischer's fortress, or Cobb's subterranean hotel suite, model the different "levels" of the mind, and the way the mind represses certain forms of knowledge.
Time
Dreams confuse the subjective experience of time in Inception in several ways. As Cobb explains to Ariadne, an hour in a dream is only five minutes in reality. When one goes "deeper" into subsequent dream layers, time becomes further distorted, as it does when the team must go three layers deep in Robert Fischer's dream in order to perform inception. Cobb confesses that he and Mal spent what seemed like fifty years in "limbo" together, before he "incepted" the idea in her to want to wake up. Dreams are also a setting where projections from one's past can continue to manifest in the present, as Mal continues to do in Cobb's subconscious. Nolan uses dreams as a way to explore the elasticity and rigidity of time—for example, although different dream layers unfold at different rates, the dreamer must be awakened with a "kick" that is synchronized simultaneously across all layers of the dream.
Guilt
Cobb's feelings of guilt over the circumstances of his wife's death provide the emotional center of the film. Specifically, Cobb feels guilty for performing inception on his wife Mal, so that she would agree to wake up from the fifty-year dream they shared together and be with their children again. Cobb's unresolved guilt in the external world leaves him vulnerable to attack in his dreams, which Mal's projection is prone to invading. The film's use of Edith Piaf's song "Ne, Je Ne Regrette Rien," ("No, I Regret Nothing") is also a reference to the theme of guilt, sung from the perspective of a woman who has decided to relinquish her attachment to the past. Like the voice of Piaf's song, Cobb eventually learns to detach himself from his feelings of guilt and regret over Mal's death.
