The film sets up a dichotomy between the inorganic—the
ship and its reactor filled with ornamentation we are
to assume is all for the realization of profit,
contrasted with its intensely human (and feline) crew.
The gorgeously-rendered hull of the ship acts as a
prison and its corridors restrict the possibilities of
its inhabitants. And yet, among the first thing we
hear from this crew is the seeds of organization, of
raging against their imprisonment.
But as soon as the film establishes this dichotomy, it
begins to play with it. The titular alien's nest is an
organic mockery of the ship, and once the xenomorph is
aboard the Nostromo it begins to cut (literally)
through the metal and plastic bonds, the hyperorganic
coming to dominate the machine. Eventually, the crew
itself begins taking action against the machine in
self-defense but from it and from their pursuer.
Ripley as a character becomes elaborated along with
this dichotomy. Although she's quiet at first, not one
of the crew who draws attention in the first few
scenes, the more we see from her the clearer her
values become. She holds life in the highest esteem,
even that of Jones the cat. In doing so, she stands in
opposition to the "purity" of force represented by
both the Nostromo and the xenomorph. In her, we see
that purity is intrinsically a false goal—the true
value is always in the impurities that make us people.