The fantastic mind: This is what's active when I read books, examine memories, do mathematics, and dream. Vision, sound, smell, texture, emotion, and kinaesthesia simulated and under some amount of control.
The word mind: Text and inflection and sound and meaning. This is what's active when I speak or sing, whether internally or aloud (and I'm more or less constantly doing one or the other when awake, usually aloud but not always).
The reactive mind: Processing inputs, forming connections, and responding to them. This is what activates during empathic conversation, when making jokes, and during most kinds of problem-solving. Mostly below the conscious level, and the responses are left to the word mind to use or not use (in conversation), or the fantastic mind to visualize and examine (in problem-solving).
The guts: Some might call this "the intuitive mind" but mine is full of crap. It gives me anxiety about things for no good reason. It also tells me to stop what I'm doing and check on time-sensitive agenda needing my attention, so I do attend when it flares up, but it's not great about giving direction to do something, just to stop or avoid things. It's like a smoke detector that goes off randomly, but also when there's smoke. No false negatives, so you keep using it, but lots of false positives.
Generally, as an introvert, the fantastic mind is active best when I'm alone or at least in calm, familiar surroundings.
The reactive mind I find somewhat draining to use but usually that's compensated for by the results: emotional connections, jokes, problems solved, recognition at work. But I don't really control its outputs, only whether or not I use them; if a task goes in and nothing comes out, that's the ballgame; I might not even remember there's a task anymore until something reminds me.
The word mind is closest to the decision-making process, and so I tend to think of it as the most "me" even though it's not fully under my control.
And the guts, well, you know how I feel about that. It may be that the guts are the same thing as the reactive mind but acting on subconscious inputs rather than conscious ones, I suppose.
Obviously a little simplistic, but those are the four primary mental modes for me.
You ask the question in the hope that the answers will provide you either entertainment or edification.
I answered the question in the hope that others will give me a sense of validation for my views or, failing that, start an entertaining or edifying discussion on a topic I'm already interested in.
Because they have enough evidence to satisfy them that they should, if they're rational; because they were taught to and never questioned it through self-examination, if they're not.
Note that evidence is not the same as proof; other users have pointed out examples of evidence such as 'testimony from trusted authorities such as parents' and 'personal spiritual experiences' and since those answers were very detailed I won't repeat them here.
I like videogame lore theorycrafting like r/TESLore but real life theorycrafting is fun, too. You would see it sometimes on r/DankChristianMemes, r/Stoicism, and lots on r/etymology. r/C_S_T was devoted to it.
Brown sauce is not nearly as popular as hotsauce where I am. I always have a bottle of A1 and a bottle of catsup, but I typically have a half dozen each of mustards and hotsauces.
Editing to add I always have a jar of homemade chimichurri ready to go as well.
looking back, it has always been some variant of "to understand and be understood by another"