Does Good Sci-Fi Need to be Realistic? (and is it really different from fantasy?)

There can be a fine line between science fiction and fantasy, so much so that they can occasionally be hard to distinguish from one another. So what makes the two genres different?

science fiction
- noun
A form of fiction that draws imaginatively on scientific knowledge and speculation in its plot, setting, theme, etc.

fantasy
- noun
Literature. An imaginative or fanciful work, especially one dealing with supernatural or unnatural events or characters.

When the two definitions are placed side by side, it seems obvious that they are different. One is based on the natural logic of the real world, whereas the other is based on unnatural logic apart from this world. But it is when we look at works from both genres that the lines begin to blur.

Do Good Sci-Fis Need to be Realistic?

For a book to call itself a Sci-Fi, I believe it's got to be more feasible than a fantasy novel. Science has it's boundaries whereas with fantasy you can create your own.

An example of a book I would call a poor Sci-Fi that doesn't quite deserve the Sci-Fi label is A Thousand Pieces of You. There is absolutely know explanation for how the scientific elements of the book work, with the narrator knowing nothing about the machine her parents created to jump between dimensions other than the fact it gets the job done. Here the rules are so limited that I'd consider the book to be more of a fantasy than a Sci-Fi, as it is largely outside the realms of logic (although personally I think that'd be a disservice to the fantasy genre).

A more loyal Sci-Fi that comes to mind is Vicious. The book mentions how people in extreme circumstances have been able to do things like lifting cars. Vicious speculates that near-death experiences produce an adrenaline rush so strong that it permanently alters the individual to have extraordinary abilities. This somewhat complies with what we know from science - just embellished with a little fiction.

I think for me to enjoy a Sci-Fi there has to be a lot of logic to it, and so I do think they've got to be realistic. If not, then it's really no different from Fantasy - the main difference being that it's more underwhelming because at least fantasies embrace the illogical nature of magic.

But is there really any difference between the genres?

The Avengers movies suggests there isn't. Characters like The Hulk and Captain America rely on science for their very existences and Iron Man depends on the technology he has created. But Thor stands apart - he has supernatural powers that have no scientific explanation, making him more of a fantastical concept than a scientific one.

The fact that the two genres coexist and often times overlap would imply that there isn't really a difference between the two. And if there is, the logical Sci-Fi elements would be watered down by the unnatural fantasy aspects, because surely the natural and unnatural cannot coexist. Easier to say that it's all unnatural - all a fantasy.

What Sci-Fis are loyal to the genre, then?

Truth be told, I haven't read many. I hate to be underwhelmed by Sci-Fis, so as much as I enjoy the genre I tend to avoid it because I cannot stand it when they are illogical or not fully explained.

Perhaps my perspective here is not so much against fantastical Sci-Fis as it is illogical fiction as a whole. I just strongly believe that books need to have the Sci as well as the Fi to deserve the label.

What are your thoughts? Is there really a difference between Sci-Fi and Fantasy, or should the boundaries be clearer? Can you tolerate Sci-Fis that aren't very feasible as long as there's a good story? Or would you just rather stick to Fantasy since the boundaries there are clear?