Definitions of ScienceFiction are quite thoroughly covered in The Encyclopedia of Science Fiction by John Clute and Peter Nicholls. All inconclusive, as one might expect, but with insights scattered about. There is a great one-liner about SF being where 'cognition meets the Gothic'. The Gothic being an even-less-regarded (even more popular, though) genre, which is if anything more baggy and helpless about self-definition, this has the effect of supporting a proposal that SF as SpeculativeFiction ought to be 'open' on one side. It is hard to see the Alien film series, as not Gothic. It is interesting to see the HAL section of 2001: A Space Odyssey as about a haunted castle, as well as explicitly about AI.
Therefore, draw an inference from La Mettrie (WikiPedia:FrenchMaterialism), proto-cognitive scientist and proponent of humans as machines, to Mary Shelley. The PrincipleOfSufficientReason hits trouble at a proverbial level: 'there's no accounting for folks'. It also of course draws the line at magic, the staple of fantasy fiction. Whether it should also draw a line at faster-than-light-travel (seven-league boots) and cryogenics (Sleeping Beauty) is a troubling question only if the proposition is that HardScienceFiction is 'realistic' (the negation of that is called TechnoFantasy here).
Authentic-seeming science, scarcely-credible people is a trade-off, and some form of AnthropicPrinciple forbids it eventually. Assume SocialMalleability exists in the face of technological change, contact with aliens or any of the other SF staples such as ark-voyages and planetary isolation; but require that it be a constraint, too, adaptation but within limits.
After all, the dark side of the moon was explored and mapped short years after it became feasible, while the dark side of humans is still very much unknown territory. Containment does seem to be a good idea: very dark SF such as CJ Cherryh's Cyteen perhaps works less well, however convincing the consistency of the genetic engineering talk, than her 30,000 in Gehenna based on encounter with reptilian intelligences.
The defensive form of this thinking is defined by Clute and Nicholls as the 'planetary romance'. We can read this as a 'possible world' that is not proposed as a possible universe, just a localised planet/system. There is some otherness at work: a mystery. Just as in the classic mystery novel, there will be an unfolding as solution; which will leave some of the central characters free to move on and the scheme of things without fundamental change. Of course the variations on this theme are worked out: what if containment fails (Dune), what if isolationism is the solution (Gordon R. Dickson's Dorsai books as capped off by The Final Encyclopedia). The Internet itself (what if we were all online all the time?), if it has done anything conceptually, seems to have broken down that assumption about cats being put back in bags: too Cold War, too 1950s.
In Deconstructing the Starships, GwynethJones? touches on what I made into AnthropologicalThoughtExperiments?; as a matter of technique for an SF writer. She writes
The creation of any sf story or novel involves the devising of a system of correspondences between a world the reader/writer knows and a world that meets the needs of the particular mental experiment in question.
In other words, deal with mostly CommonContext while highlighting and making plausible something outside it. One cannot of course stop there, as author. Cherryh employs almost as standard a plot in which a three-fold collision can be seen coming, but, just as with three billiard balls coming together almost simultaneously, not with predictable results. At a given juncture action takes over from dialectic, and novelty can arise.
Tom Shippey writing about Jack Vance:
Vance has ... colonised the entirely new space created by ... the science of anthropology, as fantasy writers have colonised ... archaeology. And: ... he has ... gone further than any other author in exploring ... issues of cultural comparison, absolute and relative value, the balance between multi-culturalism and self-respect.
This is from Jack Vance: Critical Appreciations and a Bibliography, ed. A.E. Cunningham (2000), in a paper People are Plastic: Jack Vance & the Dilemma of Cultural Relativism. Vance's speculation is more obvious than his Gothicism, though the latter is there too in doses.
There are probably numerous theoretical positions on the Gothic; one I encountered suggests that it is 'writing out the loss' (cf. WritingAgainstLoss). The Shippey article navigates to Vance's work starting with the idea of lost cultures, and suggests Vance is less afraid to invite us to judge (the Aztecs, say). If one accepts that cognition has a limit in the field of values - that some relativism about 'possible cultures' is a fact - it does seem that SpeculativeFiction can coherently be defended as a way of looking over the fence.