Your writing shouldn’t leave anyone feeling like they should scrub with pumice or commit suicide by swallowing stoats for the crime of being human, or like humans are a blight upon the Earth, or that the future is dark, dreary, evil and fraught with nastiness, because that’s all humans can do, and woe is us.
I'll boil that down to "Do not inspire loathing."
The literature of loathing, for lack of a better term for it, has a history entirely outside of science fiction. Everything from William S. Burroughs's Naked Lunch to Louis-Ferdinand Céline's Journey to the End of the Night to Jonathan Swift's satires has this reek of loathing about it — sometimes stronger, sometimes subtler. Dostoevsky's Notes from Underground was written from the point of view of someone with such loathing, although it's arguable whether the work itself is an expression of such a feeling (the author claims it wasn't, but the work itself might well disagree with him).
Within SF, though, the literature of loathing exists mostly as a form of contrast. Humanity has an unbounded propensity for arrogance and cruelty; therefore, let us show up that arrogance and cruelty on a cosmic level. Let us make humanity a victim of his own vices a thousand times over; that'll show him good. What starts as a cautionary tale often ends up as a rejection of there being anything of value in being human, or in humanity as it currently stands.
I have never found this attitude wholly incomprehensible, especially when trying to deal with some of humanity's dumber elements out on the freeway or in the supermarket checkout. Sometimes I suspect this attitude is related, however loosely, to the transhumanist notion that humanity as he exists is something which needs to be overcome (pace Nietzsche, naturally) and that the best way to avoid such contemptuous evaluation is to, well, evolve.
This notion is not new, although it is undervalued. It wasn't new when Barrows Dunham wrote in Man Against Myth (ca. 1950) about how human progress has indeed taken place against all our instincts to the contrary. Whole ways of being human have since passed under the bridge. "The medieval knight lies buried with his battle-ax," he wrote, and while there was room to lament the concomitant amount of romance that had vanished along with such a figure, there was little reason to grieve for the rest of the world that vanished along with him — a world most of us today would find uninhabitable and inhospitable. It was not a requirement to show contempt for such a figure to allow him to pass into history; all that was needed was to show a better way. Even Dostoevsky's Underground Man challenged his readers to show him a better way, which he would then adopt freely.
If you think there's something better than the humanity that we have now, it helps to point the way towards it. There are many works that capitalize on a wholly cynical view of the human condition — I outlined a few of them above — but no one thinks of them as being particularly visionary. They help us have some more vivid sense of the dimensions of the problem, and that by itself is valuable. That said, it's far harder to create such a work without simply retreading previous ground or painting one's self into a corner — just as it is also hard to create a positive vision that is not Pollyannaish, naïve, misinformed or just as much an unreachable dead end. We wince, rightfully so, at those old stories of a future with one-world governments and food pills and flying cars, because of all the things they simply couldn't know about that would make such a future untenable.
The best way to point towards some better place is to show an embodiment of it, and also to not shy away from even where it might fall short. No better future would be without its own flaws that would in turn need to be transcended. Transcendence is a process, not a singular goal.
What's more, it would scarcely help to further suggest that anyone reading is inherently unworthy of such a future by dint of being a creature of their circumstances. We are always only going to begin as beings of our moment in time and space, and the distance we travel from that point is always relative.
On a related note: There are those (like Alex Rosenberg, in The Atheist's Guide to Reality) who express disdain at best and contempt at worst for literature and storytelling as ways to both know man as he is now and man as he could be. I love science (boom-de-yada, boom-de-yada), but I cannot bring myself to understand such a viewpoint. Not all of human experience is best seen through a scientific lens, because as scattershot and messy as art can be, it provides something that cannot be had anywhere else: a way for mankind to speak in his own defense to and for his own kind. Someone who wants to speak of man without listening to him hardly seems to be up to the job.
Would a more-evolved man of the future see art as a mere effluence, a social artifact of a less-evolved self? I cannot say, but I doubt it, and I doubt further that we need to stiffen our lips and take on the manner of such an allegedly more-evolved man for our own sake. Everything I've seen suggests that an individual, and a society, with that much less in the way of creative and emotive art is that much more susceptible to emotional and, yes, intellectual barbarism. You could argue that a sentient being which has put aside emotion will have solved this problem — but again, all I've seen suggests that without emotions, you don't have a sentient being.
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