I gather that the literary sense of the three Greek words Thesis, Antithesis and Synthesis first appears in the 16th Century. They're not in Poetics by Aristotle, but I wonder if they really had earlier literary origins, because they're so fundamental, so useful. It's interesting to think about them when working out theme and structure, and in particular the need for the hero to change.
In one definition the Thesis is the core value held by the hero, the Antithesis is the core value held by the opponent, and the Synthesis is the writer's version of the world in which neither hero nor opponent wins fully, but equilibrium is nonetheless restored to the story universe through a form of compromise — possibly at a higher level than before.
Before a story gets going, the world is at equilibrium. Everyone might be unhappy but the situation is thus far unchanging, however volatile. Then the inciting incident occurs (caused by the opponent ideally) and the world is thrown into turmoil. How the pieces land and everything comes back together again is dependent upon how the characters in the world act upon the world and each other.
Conflict, verbal or physical, and all manner of Machiavellian machinations result in a new set of values becoming dominant and controlling the world of the story. Everything works itself out in the end and, if someone is still not happy to let this new equilibrium take hold, then it's not the end. Cue the sequel.
This definition of the conflict of the story being not just between two people, but between two people and what they believe in is very powerful. It allows you to know with some certainty how your characters are going to react when presented with situations that are anathema to their values.
And remember your hero isn't perfect, they have flaws. The best stories have heroes with flaws that are moral — they are hurting someone else or others through their actions (or inaction). The hero's values then are flawed, because something is holding them back from necessary change.
Whatever the desireline on the surface of the story, there is this unrecognised moral need that must be overcome in order for the hero to prevail in the surface story and achieve his desired result (except in tragedy of course). Often this transformation (Synthesis) will see the hero transcend their initial desire and now aim for something higher, and this expansion of consciousness is what allows them to defeat their opponent, and what the opponent believes in.
The other key thing that comes from thinking about Thesis, Antithesis and Synthesis is that you the author undoubtedly have a position yourself. You have your own stance, a strongly-held set of values, you have your own politics, you know what you think.
So how do you set out your argument in dramatic form? Do you just have your character rant about what you believe and have people around them passionately agree? It's a risky strategy but it has been done. In Paddy Chayefsky's Network the main character rants (live on his evening news show) about the state of society and he says, in plain language, many things that are perfectly relatable (Thesis) ... but he'd never get away with it if it wasn't set against a backdrop of a cynical corporate culture obsessed solely with obtaining high ratings (Antithesis).
In an exaggerated world of disinterest, the "mad prophet of the airwaves" is an intriguing character who wouldn't work in many other films. And in the end (spoiler alert) the TV company has him killed on air in a way that boosts ratings for one of their other shows. That's Chayefsky's Synthesis — an ironic position that the corporate mentality will use you until you're no longer useful, and doesn't care a bit about morality. In this story universe equilibrium is restored (at a lower level) and a dangerous precedent has been set. The soulless corporate mentality of mass entertainment was a big theme in the 70s, with films like Rollerball and Deathrace 2000 both making similar points.
Generally though, you might want to go for a little less polemic yourself. And maybe you don't have a major political message to get across. Maybe you just have a conflict between Loki and Thor and it's a pure, flat-out, power struggle for leadership. One guy wants to be an evil dictator and one guy wants to be a benign dictator. Or something. (I haven't fully absorbed the politics of the Marvel universe yet).
The point is that by coming up with a world view that's very different, or even seemingly opposite, to your own you create balance to your writing and you get deep value-driven conflict (drama) and rich, passionate, characters for free.
Whether your world view is the Thesis or the Synthesis (the writer's view of how a conflict should best be resolved) you'll have a conflicting opinion to give your work depth, and a lot of realism.
