"Relativism"
Relativism is the key word to understand MGSV.
As the game itself tells us "there are no facts, there are only interpretations":
this story doesn't have a completely defined narrative, it is up to us to fill in what is not clearly stated and to interpret what happens.
We choose the shape, the color and the weight of the pieces that form the final figure.
That is why Metal Gear Solid V: The Phantom Pain is unique in its genre.
It is neither a roleplay-like story nor a strictly linear movie-like story; the decision of what to belive is yours, and yours alone within a pre-existing solid narrative, the one of MGS.
Technically, relativism was also a key part for MGS3 (The Boss outright stated that friends and enemies are purely on relative terms), and also MGS2 (Snake's "no such thing as absolute reality" advice to Raiden in the ending).
Totally correct, in the MGS saga usually a theme is not "unique" of a single game, but it is present in many (if not all) of the other chapters as well.
Let's see the main theme of the "meme" for example: it's the leading theme of MGS2, but it's implicitly and heavily present in MGS1 and MGS3 as well.
Relativism, alongside existentialism, are very much present in the entire saga, especially in MGS2 and MGSV.
Personally I perceived that MGSV wants to push more on this theme compared to the other chapters: to Raiden it has been given some sort of "reality" thanks to David and Hal, he didn't know what was true and what was part of his imagination, but the "external factor" represented by Philanthropy was an anchor of safety; in contrast, Venom Snake couldn't objectively trust anyone to find a reality more correct than another. He could choose to trust some "facts" or some "individuals", but it would remain a choice in the fog, an interpretation between many possibile ones.
You as Venom Snake would feel inside "the truth that there are no truths" if not the ones you choose for yourself (existentialism), in a reality in which they try in every possibile way to keep us away from any "real" truth (relativism).
