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And in chapter 15 of the Revenge of the Sith novelization (which should actually be canon in any place that doesn't contradict the movie), Windu says:

And in chapter 15 of the Revenge of the Sith novelization, Windu says:

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:

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:

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). 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:

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: