Well let's look at each one of them in detail, as we did with the tropes.
- The setting must be coherent within itself.
This rule seems pretty obvious, and it definitely applies to other genres. In fact stated as is it would apply to any genre whatsoever. Here we would have to expand this rule, and James Gunn's proposal that a Science fiction story takes place in a world of extended every day experience fits in nicely with that need to add more detail to this rule.
- There must be a clear set of rules within the setting. (I.e, if pigs fly there has to be a rational explanation). One more we have a need to expand on this rule within the specific context of Science Fiction, for this has been argued as one of the most important aspects of the genre, especially in relation to the difference with Fantasy and supernatural genres. It could be argued that in Fantasy and Paranormal stories the rules are clear, but upon closer examination it can be seen that there are many issues whose only explanation is "magic" or some related phenomenon.
- Technological or social changes must be plausible (credible within the rules of the setting)
This rule is best explained by Farah Mendelsohn's statement that the changes must be inscribed within a "discourse of science". It does not have to be a real and verifiable scientific axiom of our real world, it just has to be written in a fashion and language resembling that of science.
This revision of the rules for deciding whether a given work is indeed SF leads to a clearer understanding of the boundaries for a very hard to define genre. However there is still some more fine tuning to be made, and that will be the subject of the next post.