As Paul Kincaid has stated, there is no ONE element that can be identified as clearly defining of the genre, but rather it is a multitude of intertwined threads, coming from a multitude of origins and sources, which define the genre.
This sits in well with the tropes and rules I have mentioned as the constitutive elements of SF. However, in order to use them as the guidelines for selecting texts clearly belonging to Science Fiction, it is necessary to refine them. Based on the expansion of the tropes to see their relationship with works of other genres, a new expanded version of them would be as follows:
- The story takes place in a different time than that of the author and in such a manner as to be clearly different from historical facts or a strict continuation of our own reality.
- The story revolves around a given "scientific" discovery or theory, whether this be a known fact at the time of the writing or an inexistent one, it has to be presented in a plausible manner. It can belong to either the "hard" or "soft" sciences.
- It involves technologies not found in our world, which can be identified as deriving from scientific theories, real or not, ans with a credible explanation.
- Extraterrestrial beings are involved. The type of being has to be understood as inhabiting a universe rationally and "scientifically" connected with the universe of the story; paranormal beings are not considered as extraterrestrial in this instance, but trans-dimensional ones (albeit in a "scientific" context) do.
- The story takes place on other planets (one or more), that can be identified as being located in our universe, even if earth is not mentioned. The story can also take place in outer space, and once again this has to be explicitly an expression of our own universe.
- Technological innovation plays a central role in the story, or as Darko Suvin has called it the "Novum". Technology is more than just a convenience used by the characters in the story, and
is in fact not found in our everyday world, and can also be a variation on an ancient technology we no longer use.
- Society changes up to a point where we can no longer recognize it as our own. More than just a change it's an evolution (which can in fact be a regression), and has to have an explanation as to how it came about, and this in turn has to comply with rationality.
- When the story takes place in time very close to our own, and in our planet, the setting for the story (backdrop)is not something found in everyday life, but has to be
identifiable as a plausible, credible, and coherent scenario.
As for the expanded rules, these should probably be more like the following:
- The setting must be coherent within itself in terms of compliance with its own physical and theoretical rules, set in the guise of real world scientific discourse.
- There must be a clear set of rules within the setting, and they must be made clear, not necessarily in full detail, and must rely on rationality for its explanation, not on faith or emotional arguments. (I.e, if pigs fly there has to be a rational explanation)
- Technological or social changes must be plausible, and be presented in such a way as to be a logical explanation of the resulting state of affairs, and respecting the rules and coherency of the setting.
These boundaries are not be all inclusive, since you can probably find stories labelled as science fiction which do not fit entirely into them. It is also worth noting that not all tropes must be found within a story for it to be science fiction, but most stories do have more then one trope; as to the rules they are mandatory, and when a story fulfills only one or two of them, then that's when problem cases arise, for they are in a harder to define situation, being at the same time science fiction and something else, be it paranormal romance o horror, and these will be the issue of a latter posting, for some of them might be suitable matter of study for a look at the relationship between science fiction and political thought, and certainly if you want to look at the much broader category of speculative fiction.