Let me explain what I mean: are the dark side and the light side of the Force just two different parts of the force with nothing between, or are they two sides of a spectrum? Can a Force-user be in between?
Legends is okay, but not preferred.
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There are a lot of different views between the Force, some which have a Light and Dark side, some which don't, and some which have some other view.
In Legends, this philosophy was called the Living Force. This view stated that the Force was divided between light and dark, with no state in between. Those who try to walk in the middle are eventually drawn to one side or the other, and the Jedi are always mindful of falling to the dark side.
Binary dark and light, no in-between.
Anakin Skywalker encountered The Ones who are a family of Force users that some believed to be god-like beings who embodied the Force, with the Son being the Dark Side, the Daughter being the Light Side, and the Father being a sort of balance keeping the two in check.
There is some debate among fans about whether or not The Ones are true embodiments of the Force, or merely a family of immortal beings who were symbolically drawn to aspects they saw as representative of the Force.
If they were true embodiments, then that would definitively describe the nature of the Force, including the fact that a "balance" between light and dark exists. But the fact that the Ones have been given a species name, and that all three died, yet the Force continued to exist, suggests that they are merely just another group of people, immortal though they may be, who have their own viewpoint of the nature of the Force and modeled themselves after it.
Regardless, their existence seemed to be relatively unknown, and Anakin Skywalker still believed in the Living Force, so this codified belief seems to be a minority view.
Binary dark and light, but there is a well-defined in-between "balance".
Adhering to a philosophy called the Unifying Force, the Force is one single entity that is neither light nor dark. It was something akin to a deity that did not take sides. Note that there does seem to be some overlap here with the Living Force; Yoda seemed to believe in the Living Force and the concepts of a Light and Dark Side, but believed that visions were neither.
No concept of dark or light exists.
Adherents to the Potentium view of the Force believed that the natural state of the Force was inherently good, and that there is no light and dark side. Rather, it is a tool and only the intentions of the users of the force to do evil that are evil. If you want to use Force Lightning all day with good intentions, that's absolutely ok. This view was heresy for the Jedi Order, since it denied that the Dark Side of the Force existed.
The Force is inherently good, only your intentions can be evil.
The opposite of the Potentium view, the Way of the Dark asserts that the Force is inherently evil, and that ultimately it trends towards destruction. This takes the view that any concepts of good are irrelevant since you are going against the natural way of the Force.
The Force is inherently evil, and good is an ultimately meaningless concept.
A view with some commonalities to the Unifying Force, the Insidious Force states that the Force is like an uncaring, sinister deity. After all, nearly every conflict in the Galaxy Far, Far Away is a result of the Force. It seems adherents do subscribe to the idea that there may be a light and dark side, but it seems to be inconsequential
The Force is inherently uncaring and sinister, light and dark may exist, but are inconsequential.
To the Aing-Tii monks, The Force is neither light nor dark, but rather a spectrum like a rainbow. In other words, you can be more light or more dark, but can also be right in the middle as well. These monks typically stayed on their homeworld, so few knew about this philosophy.
Light and dark exist, but as two ends on a spectrum.
In Disney canon, it appears that the Force is pretty definitively drawn between the Light and Dark Side, and if there is an in-between, it doesn't exist as a spectrum. In Legends, there are a lot of views that do allow for an in-between, or suggest that The Force is of a different nature.
To the first part it seems to me the dark side and light side have in themselves been presented in pretty binary terms, but to the second I would say that there is some degree of a spectrum with actual Force-users, in the sense that they can draw somewhat on emotions associated with the dark side but balance them out with light side emotions in a way that prevents them from "falling" to the dark side.
I base this answer more on what Lucas has said about the way the Force works than what has been explicitly presented in canon material (maybe someone else will be able to find some good canon sources, but Lucas is kind of the 'word of God' about Star Wars so his comments at least would seem to carry more weight than Legends material written by authors who didn't necessarily consult him). If you look at the "Behind the scenes" section of the "legends" section of the Chosen One article on wookieepedia, there are a bunch of quotes about Lucas' concept of the meaning of the dark side and the light side and the need for individuals to balance both sides within themselves. For example, at 16:40 in this interview Lucas says:
The film is ultimately about the dark side and the light side, and those sides are designed around compassion and greed. And we all have those two sides of us, and that we have to make sure those two sides of us are in balance.
"Compassion" and "greed" seems like a fairly binary distinction, but people obviously have both sides in themselves.
Likewise, at 53 seconds into this video showing a taped discussion with the writers of The Clone Wars, Lucas talks about the dark side and the light side as representing selfishness vs. selflessness, again a fairly binary distinction. But he also distinguishes between people who "balance" the two sides and people who "go to the dark side", becoming entirely fixated on selfish desires and the fear and anger that result from being worried the objects of their desires will be take away from them. Quoting from the video:
The core of the Force--I mean, you got the dark side, the light side, one is selfless, one is selfish, and you wanna keep them in balance. What happens when you go to the dark side is it goes out of balance and you get really selfish and you forget about everybody ... because when you get selfish you get stuff, or you want stuff, and when you want stuff and you get stuff then you are afraid somebody is going to take it away from you, whether it's a person or a thing or a particular pleasure or experience. Once you become afraid that somebody's going to take it away from you or you're gonna lose it, then you start to become angry, especially if you're losing it, and that anger leads to hate, and hate leads to suffering. Mostly on the part of the person who's selfish, because you spend all your time being afraid of losing everything you've got instead of actually living. Where joy, by giving to other people you can't think about yourself, and therefore there's no pain. But the pleasure factor of greed and of selfishness is a short-lived experience, therefore you're constantly trying to replenish it, but of course the more you replenish it, the harder it is to, so you have to keep upping the ante. You're actually afraid of the pain of not having the joy. So that is ultimately the core of the whole dark side/light side of the Force. And everything flows from that. Obviously the Sith are always unhappy because they never get enough of anything they want. Mostly, their selfishness centers around power and control. And the struggle is always to be able to let go of all that stuff."
Page 182 of Star Wars: The Annotated Screenplays also discusses Lucas' ideas during a certain part of the writing of The Empire Strikes Back about how Luke has to draw on the dark side to some extent when using certain Jedi abilities, but this won't cause him to fall unless he fails to balance it with the light side and allows the dark side to start "using him":
In regard to the dark side of the Force, the story meeting transcripts suggest that although one can't see it, it should be the real villain of the story. In his training Luke discovers the roots of the evil Force. The danger, the jeopardy is that Luke will become Vader, will be taken over. He has to fight the bad side and learn to work with the good side. Lucas felt that at one point during the training Ben should explain to Luke that he should use his powers with moderation. If he uses too much of the Force, it will start using him. For example, to lift objects Luke has to use the bad side of the Force, so if he overuses this power, the dark side will start taking him over as it did with Vader. When Luke fights, he has to use the dark side, but he is also using the good side for protection.
As far as Legends materials goes, there is some evidence there too that individuals can draw on both dark and light sides without falling to the dark side. For example, look at the wookieepedia article on the Gray Jedi, which in one of two uses of the term can refer to "Force-users who walked the line between the light and dark sides of the Force without surrendering to the dark side". And the "vaapad" lightsaber technique developed by Mace Windu in Legends, also known as Form VII lightsaber combat, was said in a few books to draw on "dark" emotions like anger and the desire to crush one's enemies, and use them in the service of the light. For example, in chapter 5 of the book Shatterpoint Mace wrote in his journal of feeling a dangerous "blood fever" during a particular battle, "the lust for victory. To win, at any cost." He also wrote "It's not overpowering; I haven't fallen that far. Yet. It's more a preference. An expectation. An anticipation that has been disappointed." And of Vaapad, he wrote:
My style grants great power, but at a terrible risk. Blood fever is a disease that can kill anyone it touches. To use Vaapad, you must allow yourself to enjoy the fight. You give yourself to the thrill of battle. The rush of winning. This is why so few students even attempt the style.
Vaapad is a path that leads through the penumbra of the dark side.
And in chapter 15 of the Revenge of the Sith novelization, Windu says:
"I created Vaapad to answer my weakness: it channels my own darkness into a weapon of the light."
It is a spectrum.
There is one Force, the difference is how you approach it, what you want to get out of it.
The light side focuses on controlling selfish desires for the benefit of everyone.
The dark side focuses on gathering as much power as possible for the benefit of the individual with that power.
Because The Force represents so much power, it is almost impossible to use it in a non-selfish manner unless you have a strong motivating reason not to. A good analogy is a $1,000,000 daily allowance. What would you do with it? Would you use it to get what you want? to help people? to steal things you can't buy? The Jedi believe that people, especially those who cannot control their emotions and desires, will eventually start using their power for selfishness. This is why they always talk about the light and dark side as absolutes, it is almost impossible to be neutral.
Anakin is a good example. While he wanted to do what was right, his primary desires were to get what he wanted. This was shown when he gets angry about how long it took to get on the Jedi council, when he attacked the sand-people villagers when his Mom died (rather than finding out why they were acting strange) in order to exact his revenge, and the whole ending of Episode III. He knew what he wanted and he did not care who he hurt or the amount of damage he would do, as long as he was able to get what he wanted. It was his strong, uncontrolled emotions which pushed him to fight so hard for what he wanted (despite the destruction), which is why strong emotions like love, hate, anger, envy, and fear are counseled and guarded against by the Jedi, but embraced by the Sith.
One additional "side" is mischief. While not as strong a drive for most humans, a Loki or Puck character who acts to bring change and "keep things interesting" would not fall into the self serving nature of the dark side or the selflessness of the light side. Those in this category would technically be gray since they can do light and dark side things, but they are not defined by those actions (their light or dark side choices cannot be used to predict future decisions).
'The Force' is an absolute in itself, like the Daoist Dao. But we can be in touch with it, and be at one with it to varying degrees. Therefore we have a 'paduan learner' or a master, like Yoda. These ideas are just 'transliterations' and re-packaged versions of ancient Daoist or Buddhist philosophical concepts.