Fantasy and the fantastic

I proposed a rule of thumb for distinguishing between fantasy and SF. It's descriptive rather than prescriptive:

SF tells you that in certain circumstances, the laws of nature could operate in certain way, and invites you to set aside your disbelief; fantasy concedes that the laws of nature do not operate in a certain way, and invites you to imagine how it might be if they did:

durham_rambler points out that SF is not necessarily concerned with the impossible, only with the not yet possible:

Given that scientific knowledge is always growing, there are always going to be writers who write speculative fiction that postulate new discoveries that are consistent with the laws of nature as we know them.

Meanwhile, over at matociquala's comments, adrian_turtle makes a case for reading the Aubrey / Maturin books as science fiction:

It's science fiction where the science is navigation, natural philosophy, and political science of 200 years ago.

What connects these two points in my disordered brain is that ( speculative fiction is in the eye of the beholder. )