Science
Science is a central theme in Alien, and the film examines science's capabilities as well as its limitations. Alien takes place in a world defined by its incredible scientific achievements, exemplified by effective space travel and technological advancements like artificial intelligence. Science is what has allowed the crew members of the Nostromo to do their work, and it provides many options for them in times of danger. However, science also falls short of the crew members' needs in many ways also. While the pursuit of scientific knowledge is what has allowed for so much progress and achievement, it is also the pursuit of scientific research that motivates Ash (and by extension, the corporation) to bring the alien on board and put the lives of the crew members at risk in the first place. As the Science Officer, Ash is clinical and straightforward in his approach to the threat of the alien, and it is his belief in the purity of science, its objectivity, that allows him to ignore the human interests of the crew members, and keep the alien alive despite its violent tendencies. Scientific progress—in this case, the acquisition of an alien life form—takes precedent over human interest. Thus, science can be seen as being at the root of the primary conflict in the film. While science motivates humans to pursue progress at all costs, this is not always in the human being's self interest.
Technology and its limits
Technology and its limitations also make up the thematic bulk of the film. Technology, the use of scientific tools for strategic benefit, is helpful only to a point in Alien. Throughout, the crew members of the Nostromo, and finally, Ripley, left to her own devices, must remain crafty and cannot rely solely on the help of the ship's technological attributes.
Technology's limits are a direct result of corporate interest. In perhaps the most shocking part of the film, we discover that "Mother," the technological system powering the entire ship, is programmed to help transport the alien back to Earth, even if that means sacrificing the lives of the crew members. While the crew members had hoped that they might be able to utilize technology to dispose of the alien, the ship's technology has been locked and programmed to keep the alien alive. Additionally, it is revealed that Ash himself is a piece of technology, an android with an ulterior allegiance to the corporation. In this revelatory moment, technology both helps and hinders; the use of technology ("Mother") is what leads Ridley to realize Ash's motives, but it cannot adequately prevent or extinguish threats, and in fact, it only makes things worse.
While technology is ultimately what allows Ripley to conquer the alien—the engine of the ship is what destroys it in the end—it ultimately proves damaging and inconvenient. Indeed, Ripley must destroy the Nostromo entirely in order to escape to safety.
Fear & Panic
Alien is, first and foremost, a horror movie, and as such deals with the themes of fear and panic throughout. While the theme of science concerns a grasp of the known, horror is defined by a battle with the "unknown" and the quest to survive horrifying circumstances. In spite of all of the technological advancement and competence of the crew members, there remains throughout a "thing that goes bump in the dark," the alien. The alien threatens to kill all of them, and its powers and innately violent actions make it a horrifying evil. Throughout the film, the viewer sees various crew members contend with their fear of the alien, and pass through states of panic and alarm. The alien itself is a silent killer, unpredictable, grotesque and unknown, and it seems to descend on its victims without any warning. Thus, the alien itself represents its victims' fear, in that it is disruptive, unpredictable, and unknowable. Indeed, the moments of violence in Alien are somewhat infrequent, and much of the terror of the film lies in the crew members' (and the viewers') anticipation of the violence and horror to come.
Humanity & Morality
When Ash is revealed to be an android, it becomes clear that his mission is to return the alien to Earth, and that he has no interest in helping his human counterparts stay alive in the face of the blood-thirsty alien. As an android, Ash is devoid of human emotion or a sense of subjectivity or ethics, and he mocks his human companion's desire to behave morally and defeat the alien. He describes his admiration for the alien to Lambert, Parker, and Ripley, saying, "You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility." When Lambert accuses him of admiring the alien, Ash cannot deny it, saying, "I admire its purity. A survivor...unclouded by conscience, remorse, or delusions of morality." These attributes are what define the non-human, and to Ash, an android, they are admirable traits.
Thus, Alien defines humanity and its qualities as being connected to "conscience" and "morality." Indeed, the central conflict is between the non-human alien, ruthless and violent, and the frightened humans, who seek to do the right thing and take care of their companions. The fight to stay alive is the fight for the survival of humanity, even in the face of cold-blooded, sociopathic androids, corporate interest, and the single-minded brutality of the alien species.
Teamwork & Togetherness
The survival of the humans on the Nostromo is centered around their ability to work as a team. Even before the alien invades the ship, the crew members have explicit roles that allow them to work together effectively as a team. Parker and Brett are technicians and engineers, charged with maintenance on the ship, Dallas is the captain, Lambert the navigator, Ash the scientist, and Kane and Ripley are warrant officers. Teamwork is integral to the effective management of the ship, and Ripley invokes its importance even as people begin dying and prospects begin to look bleak. After the death of Dallas, Ripley takes charge and urges Parker, Ash, and Lambert to keep their wits about them to help her kill the alien. Frightened and unsure, they begin to question their ability to survive, which leads to infighting, selfishness, and distrust, but Ripley becomes impatient, insisting, "We have to stick together!" Togetherness is what allows them to fight the alien and help each other in dire situations. The true horror of the film, however, comes after everyone has been killed and Ripley is left to fend for herself. By the end of the film, Ripley is left with only the ship's cat, Jones, as her companion, and teamwork is no longer an option.
Survival
When Ash details his admiration for the alien, he discusses its singleminded desire to survive as what sets it apart from the human race. He defines that motivation to live as integral to the "purity" of the creature. Indeed, survival is an important theme in the film, and is what creates the conflict between the alien and the humans. Both the alien and the crew members want to survive and make it back to Earth, which creates a violent struggle to the death. Indeed, the alien is a survivor, eluding all efforts to extinguish it, its superior evolution making it primed for survival. One of the most shocking moments in the film comes when Ripley boards the escape shuttle, believing she has left the alien behind on the self destructing Nostromo, only to find the creature hiding against the wall of the shuttle. Then, when she pushes it out the hatch into space, it manages to hold on for dear life. The alien is bent on survival, invulnerable to attack until its final moments, when Ripley blasts it away with the force of the engine.
While Ash makes a case for the alien being superior because of its will to survive, Ripley represents the human will to survive and ultimately wins out. She is indeed unmoored by the presence of the alien and the horrors it wreaks, but that does not deter her from fighting back and surviving. She too is a survivor, an undeterred fighter and resilient champion.
Authority & Gender
While it is not made explicit in the screenplay, gender and its relation to authority and heroism is an important theme of the film, and Alien is celebrated by many to this day for its progressive and empowered depiction of its female protagonist, Ellen Ripley. Ripley's authority is undermined by her male counterparts from the start of the film. While she has the authority to order that Kane be kept in quarantine following the alien attack, Ash and Dallas undermine this authority in favor of bringing the alien aboard. Then, when Ash argues that he wants to keep the allegedly dead alien on the ship to bring back to Earth, neither Dallas nor Ripley want to do so, but Dallas sides with Ash anyway. Ripley must believe in her own strength of judgment throughout, in spite of being continually undercut by male dominance. Ripley's self-assurance only serves her well, however, as she remains the sole survivor of the Nostromo, and manages to conquer the alien using her wits. Ripley is believed to be the first action heroine of cinema, a resolute and resilient fighter who is never victimized by her circumstance, but fights back bravely in the face of every challenge.
