Since its premiere, 2001: A Space Odyssey has been analyzed and interpreted by professional film critics, amateur writers and science fiction fans, virtually all of whom have mentioned its deliberate ambiguity. Questions about 2001 range from uncertainty about its deeper philosophical implications about humanity's origins and final destiny in the universe, to interpreting elements of the film's more enigmatic scenes such as the meaning of the monolith, or the final fate of astronaut David Bowman. There are also simpler and more mundane questions about what drives the plot, in particular the causes of Hal's breakdown (explained in earlier drafts but kept mysterious in the film).
Stanley Kubrick encouraged people to explore their own interpretations of the film and refused to offer an explanation of "what really happened" in the film, preferring instead to let audiences embrace their own ideas and theories. In a 1968 interview with Playboy magazine, Kubrick stated:
You're free to speculate as you wish about the philosophical and allegorical meaning of the film—and such speculation is one indication that it has succeeded in gripping the audience at a deep level—but I don't want to spell out a verbal road map for 2001 that every viewer will feel obligated to pursue or else fear he's missed the point.
In a subsequent discussion of the film with Joseph Gelmis, Kubrick said his main aim was to avoid "intellectual verbalization" and reach "the viewer's subconscious." However, he said he did not deliberately strive for ambiguity—it was simply an inevitable outcome of making the film nonverbal, though he acknowledged this ambiguity was an invaluable asset to the film. He was willing then to give a fairly straightforward explanation of the plot on what he called the "simplest level," but unwilling to discuss the metaphysical interpretation of the film which he felt should be left up to the individual viewer.
For some readers, Arthur C. Clarke's more straightforward novelization of the script is key to interpreting the film. Clarke's novel explicitly identifies the monolith as a tool created by an alien race that has been through many stages of evolution, moving from organic form to biomechanical, and finally achieving a state of pure energy. These aliens travel the cosmos assisting lesser species to take evolutionary steps. Conversely, film critic Penelope Houston wrote in 1971 that because the novel differs in many key respects from the film, it perhaps should not be regarded as the skeleton key to unlock it.
Multiple allegorical interpretations of 2001 have been proposed, including seeing it as a commentary on Friedrich Nietzsche's philosophical tract Thus Spoke Zarathustra, or as an allegory of human conception, birth and death. The latter can be seen through the final moments of the film, which are defined by the image of the "star child," an in utero fetus that draws on the work of Lennart Nilsson. The star child signifies a "great new beginning," and is depicted naked and ungirded, but with its eyes wide open. Leonard F. Wheat sees Space Odyssey as a multi-layered allegory, commenting simultaneously on Nietzsche, Homer, and the relationship of man to machine.
The reasons for Hal's malfunction and subsequent malignant behavior have also elicited much discussion. He has been compared to Frankenstein's monster. In Clarke's novel, Hal malfunctions because of being ordered to lie to the crew of Discovery and withhold confidential information from them, despite being constructed for "the accurate processing of information without distortion or concealment". Film critic Roger Ebert wrote that Hal, as the supposedly perfect computer, actually behaves in the most human fashion of all of the characters.
Rolling Stone reviewer Bob McClay sees the film as like a four-movement symphony, its story told with "deliberate realism." Carolyn Geduld believes that what "structurally unites all four episodes of the film" is the monolith, the film's largest and most unresolvable enigma. Vincent LoBrutto's biography of Kubrick says that for many, Clarke's novel is the key to understanding the monolith. Similarly, Geduld observes that "the monolith ... has a very simple explanation in Clarke's novel," though she later asserts that even the novel doesn't fully explain the ending.
McClay's Rolling Stone review describes a parallelism between the monolith's first appearance in which tool usage is imparted to the apes (thus 'beginning' mankind) and the completion of "another evolution" in the fourth and final encounter with the monolith. In a similar vein, Tim Dirks ends his synopsis saying "[t]he cyclical evolution from ape to man to spaceman to angel-starchild-superman is complete."
The first and second encounters of humanity with the monolith have visual elements in common; both apes, and later astronauts, touch the monolith gingerly with their hands, and both sequences conclude with near-identical images of the Sun appearing directly over the monolith (the first with a crescent moon adjacent to it in the sky, the second with a near-identical crescent Earth in the same position), both echoing the Sun–Earth–Moon alignment seen at the very beginning of the film. The second encounter also suggests the triggering of the monolith's radio signal to Jupiter by the presence of humans, echoing the premise of Clarke's source story "The Sentinel".
The monolith is the subject of the film's final line of dialogue (spoken at the end of the "Jupiter Mission" segment): "Its origin and purpose still a total mystery." Reviewers McClay and Roger Ebert wrote that the monolith is the main element of mystery in the film; Ebert described "the shock of the monolith's straight edges and square corners among the weathered rocks," and the apes warily circling it as prefiguring man reaching "for the stars." Patrick Webster suggests the final line relates to how the film should be approached as a whole, noting "The line appends not merely to the discovery of the monolith on the Moon, but to our understanding of the film in the light of the ultimate questions it raises about the mystery of the universe."
The film conveys what some viewers have described as a sense of the sublime and numinous. Roger Ebert writes in his essay on 2001 in The Great Movies:
North's [rejected] score, which is available on a recording, is a good job of film composition, but would have been wrong for 2001 because, like all scores, it attempts to underline the action—to give us emotional cues. The classical music chosen by Kubrick exists outside the action. It uplifts. It wants to be sublime; it brings a seriousness and transcendence to the visuals.
In a book on architecture, Gregory Caicco writes that Space Odyssey illustrates how our quest for space is motivated by two contradictory desires, a "desire for the sublime" characterized by a need to encounter something totally other than ourselves—"something numinous"—and the conflicting desire for a beauty that makes us feel no longer "lost in space," but at home. Similarly, an article in The Greenwood Encyclopedia of Science Fiction and Fantasy, titled "Sense of Wonder," describes how 2001 creates a "numinous sense of wonder" by portraying a universe that inspires a sense of awe, which at the same time we feel we can understand. Christopher Palmer wrote that there exists in the film a coexistence of "the sublime and the banal," as the film implies that to get into space, mankind had to suspend the "sense of wonder" that motivated him to explore space to begin with.