Categorising "speculative" fiction

I was in a recent conversation about what classed as 'speculative fiction' and how it was different to science fiction. This followed an observation that when established authors decide to write stories set in the future they prefer to describe it as speculative fiction rather than science fiction. 

To me there is a difference. Speculating on how the world might turn out doesn't necessarily relate to changes in our scientific understanding of the world, which is a cornerstone of science fiction, nor does it depend on the ability of humans to deploy science to push back boundaries.

There was also a question of alternate history (alt-hist), which is also 'speculative', but that seems quite clearly defined already so we can save the term 'speculative' to apply to stories of a possible future instead of a different version of now. 

A suitably spacey photo...

I also divvy up sci-fi into science fiction and space fantasy (also occasionally called 'space opera'). This roughly follows the rules of 'hard' and 'soft' science fiction, although I'm not keen on those distinctions. They are dated and don't really capture the intersections of scientific progress.

The old rule used to be that 'hard' science-fiction bent or changed one rule of science, while in 'soft' science-fiction, pretty much anything goes. Keeping the former as science fiction, I'd describe space fantasy as advanced technologies separated by vast time periods from our present, or unconnected to current human history at all, like the Star Wars films being set 'a long time ago in a galaxy far, far away'. 

Here are some examples of books I've read in the four rough categories...

Alt-hist - changes details in the past to create a different set of events, or a different contemporary reality. The Plot Against America by Philip Roth explores how the USA could have adopted Nazi ideology in the 1930s. I'd also include Jasper Fforde's books about Thursday Next here and any other books about a world that looks a bit like ours but is different in some way.

Speculative fiction - starts (broadly) with where we are now and predicts what might happen in the future. This can drift towards science fiction when something novel is introduced (like triffids, for example) but the emphasis is always on the human side of things rather than the technology. Well known examples would be The Handmaid's Tale, or Nineteen Eighty-Four and Brave New World (in a previous generation). JG Ballard's books The Drowned World and The Drought fit more here rather than in science fiction.

Science fiction - stories that revolve around or focus on a particular discovery, advance in knowledge or change in technology. 2001: A Space Odyssey and I, Robot fit here because humans discover or introduce something that changes the world. I feel this is getting rarer as a genre in terms of new releases. However, the Expanse novel series is a good example of how this genre can still produce new compelling stories.

Space Fantasy - examples would be Dune by Frank Herbert, the Culture novels by Iain M Banks, novels set in the Warhammer 40,000 universe, and several of Alastair Reynolds's books. I enjoyed reading Adrian Tchaikovsky's books Children of Time and Children of Ruin, and both of those belong in this category. 

These are just my own categories and I'm probably inconsistent. But then I'm only classifying books for my own enjoyment, so it probably doesn't matter which category books end up in.