The definitions just keep on rollin’... Here are the latest evolutions of my twin definitions of science fiction, fantasy, and alternate history, the three “sibling” genres of imaginative fiction:

Part one: definitions based on the form of the fiction:

> A science fiction novel accepts ALL the scientific axioms of our real world, but adds one or two additional axioms to them.

> A fantasy novel accepts MOST of the scientific axioms of our real world, but replaces one or two with imaginary axioms.

> An alternate history novel accepts MOST of the historical facts of our real world, but replaces one of two with imaginary facts.

Thus, in brief, a science fiction novel does not violate any of the real world’s tenets, but adds additional ones, such as space travel, futuristic life, alien beings, etc.

Fantasy basically accepts magic and/or the supernatural in addition to the majority of our real world’s tenets.

But what about alternate history? The problem is that alt hist does not really fit either of the above definitions. I could have cheated and simply changed the first definition to say "A science fiction novel accepts all the scientific axioms and historical facts of our real world, but adds one or two additional axioms to them." But this would be cheating since alternate history does not really add any additional axioms to our real world. It merely extrapolates from a changed historical fact, adding no axioms at all.

Part two: definitions based on intent:

> Science fiction is the study of plausible historical change upon the world as we know it.

> Fantasy is the study of impossible historical change upon the world as we imagine it.

> Alternate history is the study of hypothetical historical change upon the world as we remember it.

This definition depends on the intent of a science fiction story rather than how many real world axioms it accepts. Admittedly, not all imaginative fiction deals with change, since much of it is merely adventures or romances or thrillers based on f&sf or alt hist settings, so this definition is more exclusive than the first one is. In fact, it is safe to say that only “ideal” imaginative fiction falls into one of these three latter categories, with “ideal” defined by Sturgeon’s Law as the desired 10% of all imaginative fiction.

Obviously, all such definitions are intrinsically “working” definitions, so any comments you might have on them is welcome.