[NB: It’s Saturday morning, and I’m at the tail end of a week’s vacation, so I’m digressing from the usual fare to write about a movie. Apologies for the distraction.]

The “science” part of science fiction is important. Good science fiction employs a reality we recognize, but with specific things tweaked or altered. These alterations serve as a way to explore how those things shape our present reality, and how the future—or some counterfactual past or present—might differ. As Philip K. Dick suggests, good science fiction isn’t fantasy, it’s futurism.

For this to work, the alterations have to be scientifically plausible. Fusion reactors are interesting, because they connect to rules and theories we know and are therefore imaginable. Magic energy cubes, on the other hand—things like the Tesseract in the ho-hum Avengers franchise—are frustrating, because they aren’t connected to our lived existence. (Equally important from a narrative perspective, magic energy cubes don’t impose any constraints on the action that we can recognize ahead of time and therefore can’t serve as sources of tension and suspense.)

The elegance of science lies in its simplicity. The basic components and rules are simple; the complexity we inhabit emerges from their interaction and combination. All the things we see around us are composed of few dozen elements, which are, in turn, composed of a handful of subatomic particles. Combinations of simple geometric elements produce dazzling fractal forms that appear over and over again in nature, often in very different contexts. Sentience emerges from clusterings of relatively simple cells—or, perhaps, circuits.

Starting from these premises, I was frustrated and disappointed by Prometheus, the recently released prequel to Ridley Scott’s 1979 science-fiction masterpiece, Alien. I was only nine years old when Alien first came to theaters, so I didn’t much appreciate it at the time. As a Star Wars fan, I was happy to go to any movie set on a spaceship, but I had to head for the lobby after the “indigestion” scene in the galley and only saw bits of the rest of the film through the doors at the back of the theater.

Going back to Alien as an adult, though, I’ve become a big admirer. I don’t much like horror films. What makes Alien so appealing to me now is its smart application of one of one of science’s most elegant theories—evolution—to answer a profound philosophical question. On Earth, humans seem (to ourselves, anyway) to stand at the top of the pyramid, so powerful that we can build ships to expand our reach into space to scavenge and prey elsewhere. As soon as we do that, though, we also expand the circle of our existence into a wider universe in which other creatures might occupy similar niches on other planets. In this wider circle, predator can become prey, parasite can become host, and natural selection can take new turns. Humans become more evidently animal and are clearly maladapted to many of the new environments in which they might land. The stories we tell about our special place in the universe crumble in our first encounters with species from its other corners. The common denominator for life in this wider perspective is the urge to survive and reproduce, and on this larger scale, we may not be so great at it after all. If the question is “What makes us humans special?”, Scott’s answer is a resounding “Not much.”

And then we get Prometheus. Where Alien relied on the elegance of a simple scientific idea, Prometheus injects narrative complexity and scientific ambiguity in a plot that grabs from biology and evolution but ultimately rejects them. The long evolutionary line connecting humans to primordial goo on Earth is cut, replaced with the quasi-mystical idea of a process begun much more recently by Engineers whose own origins are left unexplained. The simple biological imperative of survival through predation is supplanted by a worn and ambiguous political story about warring societies and weapons of mass destruction.

Good science fiction is really hard to write. Most attempts I’ve seen or read have foundered or failed because they have interwoven so many inventions and ideas that we can’t follow the threads back to our lived existence. The core of science is the experimental method, and one of the guiding principles of that method is control. To answer questions about cause and effect, you focus your gaze by varying the hypothesized cause while holding confounding factors constant.

Good science fiction often does the same. Alien puts humans in deep space, hypothesizes about what might have evolved in those strange (to us) environments, and runs one trial of an experiment to see which species would survive by dominating those encounters. (In repeated iterations of this experiment, my money’s on the aliens, the lazy digressions of the sequels notwithstanding.) Prometheus replaces this elegant design with mysticism and hand-waving more characteristic of religion than science, and it’s a much poorer film for it.