Note: This is my artistic interpretation of a debate with a subjectivist; no quotes here were said here by any real-life person.
Subjectivist: Belief systems are ultimately arbitrary, including any so-called “rational” belief system.
Me: I disagree, and would like to demonstrate that you are wrong.
Subjectivist: Fine, but just remember that you must answer any question put to you as is, and may not ask any questions about the question. These are the objective rules of debate for all time that God has told me in my own ear. Socrates be damned!
Me: Well, I don’t accept…
Subjectivist: I knew it! You’ll dodge any substantive question I ask. I win. QED.
Me: But it’s commonly recognized that there can be illegitimate questions, such as: “When did you stop beating your wife?” Such questions don’t admit a simple answer, right?
Subjectivist: <pretends not to hear>
Me: Hmmm. Well, regarding your original remark, if you look at any standard definition of “arbitrary”, you’ll find that it is rather the opposite of “rationally justified”, so you calling a rational system “arbitrary” is silly.
Subjectivist: Oh yeah? Well answer me this: Why be rational?
Me: What do you mean by “why?”
Subjectivist: Aha! There you go with your dodging!
Me: The word “why” has several senses and I can think of at least two reasonable ways of interpreting your question. Also, people can sometimes invent idiosyncratic terms. I just want to be clear on what question it is you want me to answer.
Subjectivist: Nope loser, you lost the debate. Them’s the rules! Just answer the question, or admit you lost, or be a dishonest loser (which is what I really expect of you).
Me: But it seems transparently fair to ask for the meaning of the question… I guess I’ll just assume you mean “why” as in “explain.” Notice that by “explain” we ordinarily mean “give an answer that would satisfy our need for logic and evidence, i.e. give a rational explanation.” So, this question presupposes the assumption it wishes to challenge, a logical fallacy.
Subjectivist: Assume? Did you never hear the saying “don’t make an A-S-S out of U and M-E?” Stop assuming for me and just answer the question.
Me: I don’t know what your question is.
Subjectivist: Just answer. I don’t want your questions or complaints.
Me: Well I disagree with you about everything above, including about whether I’ve rationally answered you. I have nothing else to add. Bye.
Subjectivist: OK fine. I’ll answer you just this one time, but don’t you ever be so impertinent as to ask another question ever again. By “why” I mean “for what purpose should we be rational?”
Me: I think the purpose of rationality is to find the truth. It is for the sake of truth that we choose to be rational.
Subjectivist: Why pursue the truth?
Me: Do you want a true answer to that question? Or no?
Subjectivist: God dammit! I told you that was the last stupid question of yours I’d answer. You’re no Socrates so stop playing at it. Your question is beside the point, just answer mine or admit defeat.
Me: Actually it’s the heart of the point, insincere communication is pointless. There’s several tacit assumptions to every philosophic communication, sincerity is one, which is implemented by transparency of motives. I don’t think it should be hard to admit “Yes, I want the truth.” It’s obviously harder to admit “I don’t want the truth, I just want to believe I won the argument.”
Subjectivist: You’re the one lacking in sincerity, such as by not answering my simple question.
Me: There is really no such thing as a “simple question”; all questions contain presuppositions. Your presuppositions can be different from my presuppositions. For example, one of mine is that it’s legitimate to inquire as to the meaning and legitimacy of a question, and you arbitrarily prohibit such inquiry. That’s a non-simple thing that is presupposed by our engagement; our opposite answers on this indicate that this is where we should focus our disagreement on.
Subjectivist: Fuck you “Socrates.” That’s just more distraction and dodging. I can’t believe you think you know anything about philosophy. What a pathetic joke.
Me: Well I’ll just point out that if you wanted a true answer to your last question, then your question is holding my answer to the very standard you seek to refute. But if you regard reason (i.e. legitimate arguments aimed at the truth) as “arbitrary”, then any demands you place on anyone to follow reason are illegitimate.
You should decide whether truth is something you want or not. Once you accept that there is such a thing as “humanly knowable truth,” then you’re inevitably going to have to reject any idea that “reason is arbitrary.” Rather, the truth is that there are ultimate rational presuppositions we call “axioms.” Given that these are logically presupposed by reasoning, they aren’t arbitrary. Something “arbitrary” would be something we just made up on a whim, not something we used a rational process of inference to discover. You should use the right word for the right thing, instead of arbitrarily decreeing things “arbitrary.”
Or, to be consistent at this step you might say that “there is no such thing as truth.” Then, I will naturally inquire as to whether that statement is true or not. If you say that your statement is “true”, then you are saying it is false. It is indeed false, so that would be true.