In Real World Terms, a character knows... (re-eximine the gurps description)
- Theology - because almost every character has a religion. Those who do not have a religion, have a Philosophy. In a setting with agnostics, atheists, anti-theists, and objectionists (Ayn Rand) their motives and beliefs/unbeliefs that shape a character's psyche is typically from theology. In this new global world, we are more aware of the key differences more than ever, and it provides valuable context in the game where you are trying to figure out how the other person is thinking. The key differences when dealing with a Religious Apologist vs a Fundamentalist; A Pragmatist, vs an Objectionist; Deist vs a Theist.
- Literature - It is more ingrained in our language and how we communicate ideas. Growing up with a different set of stories from someone else marks the visibile differences in culture. In the way we speak, and how meme work, metaphors, similes and all those language tools that allow us to communicate concepts too difficult to break down in a few words. In a game, it allows you to determine where someone is from despite their mastery of the language.
- History - who doesn't know a bit of their own culture's history. Like literature, it is an important marker of cultural background but history is also important because of fact finding relevance.
- Savoir-Faire - manners and how to act before their peers, lessers, and betters. this is something that can be communicated with just a glance. Un-equipped a character automatically stands out. Even lower leved folks have their own manners, which makes anyone alien to it stand out.
What is the difference between someone who uses Rationalism and some other methods like Philosophy or Theology? - there is a difference between how each come by their conclusions, in terms of accuracy only one method can be verified (rationalism/empirical method).
Characters who use Philosophy and Theology can manipulate the concepts and ideas to suit their purposes. In an argument, use the appropriate Philosophy or Theology. In a Rationalistic Argument, use Philosophy: Rationalism/Empiricism along with the Empirical Data that the PCs have found/collected.
In very strategic games, where characters are fact finding and trying to generate solutions - Philosophy: rationalism (or its earlier iterations like Pragmatism and Empiricism) gives the Players a TIP from the GM when they cannot make heads of the clue.
Compared to RAW intelligence, Rationalism/Empiricism is a method. It is an assumption that high GURPS intelligent characters are Rational/Empirical in their methodology. Especially since there exists a skill that allows for the training of the methodology.
Comparing a Character who uses rationalism vs a character who isn't, has a telling psychological difference in certainty and how they use their knowledge. A rational character is not that trusting about his own experience and certainty, unless there are ways to verify it outside his own conclusions.
In Gametheory, and in Games, the Way a NPC and PCs behaves is based on the certainty and accuracy of their information.
Not all Philosophies are learned in the Classroom. many great characters and people of history were Pragmatists (an earlier incarnation of Rationalists) who lean on their own understanding and conclusions. They are not easily swaded by hearsay, they tend to have a higher Psychology, Detect Lies skill.
Another use of Philosophy/Theology is understanding key points of difference that lead to conflict. Roll Philo/Theo to see if your gonna have problems with this person in the foreseeable future working together.
Psychology (along with Philosophy) allows a person to deduce another character's Philosophy/Theology based on their actions and credible intentions.