What makes some counterfactuals interesting and others too absurd for consideration? In Godel, Escher, Bach: An Eternal Golden Braid, Douglas Hofstadter explores this question by borrowing the concept of frames and nested contexts from the field of artificial intelligence.
In frame language, one could say that mental representations of situations can be represented by a hierarchal model of nested frames, each of which contains a set of default expectations, as well as knowledge of its limits of applicability, and heuristics for switching to other frames in case it has been stretched beyond its limits of tolerance (645).
It is only at particular levels of depth in the hierarchy, within certain bounds of tolerance that an imaginative move can be made --- a variable can only 'slip' so far. The imaginative moves in Star Trek, for example, operate on the level of politics, technology, and geography, but assume a certain base-level similarity to our reality in their 'humanness'. We can easily relate to the crew members' modes of interaction and their social dynamics. And, at a more fundamental level, our conception of causality and temporality is maintained in the Star Trek universe. The higher-level the frame, the less tolerance it allows for imaginative play in fiction. Fantasy fiction, in contrast to science fiction, allows a few more variables to slip but is ultimately subject to similar constraints.
The linguist George Steiner writes in After Babel,
Hypotheticals, 'imaginaries', conditionals, the syntax of counter-factuality and contingency may well be the generative centers of human speech. No less than future tenses to which they are, one feels, related, and with which they ought probably to be classed in the larger set of 'suppositionals' or 'alternates', these 'if' propositions are fundamental to the dynamics of human feeling...
Ours is the ability, the need, to gainsay or 'un-say' the world, to image and speak it otherwise... We need a word which will designate the power, the compulsion of language to posit 'otherness'.... Perhaps 'alternity' will do: to define the 'other than the case', the counter-factual propositions, images, shapes of will and evasion with which we charge our mental being and by means of which we build the changing, largely fictive milieu of our somatic and our social existence (232).
Must an imagined world remain plausible? What kind of limitations does this impose on our imagined worlds? And how can the choice of expressive medium influence these boundaries? (e.g. how is imagination different in animation, which allows the director to depart from human actors and physical sets; or in painting, which disposes of a linear sense of time?)