I think those principles are:
- There are no unanswerable questions
- There are no unquestionable answers — Pfhorrest

I reckon these to be the derivatives of the first principle, which in turn presupposes a third derivative: the goal. The goal in itself might be and optimally is a first principle to any branch of philosophy. In conclusion; the first principle of all philosophy is its common denominator. It can be anything, but it's common denominator is that it is its common denominator.

The higher our regard, the clearer our answers, all the while more difficult to ground in the denominator. To be that ground is God's purpose - the denominator of the denominator of the denominator. Although God is not the purpose, the purpose is reasonably confused as God. The philosophy goes above that, because the purpose is real. We have philosophy because it is real; in contrast to science which can only address that which is fleeting to some degree. This though is where science and philosophy meets. They have a common common denominator. Although philosophy can reach a little further. It touches religion where science can not.

Their common denominator is magic - my favourite philosophical topic. I believe it does better philosophically than religiously. I appreciate it from either direction though, but I don't talk about it religiously. Religion has unrefutable answers about it, but it is philosophy that ultimately makes it pristine. Either way - any first principle: common denominator.

Those who cannot remember the past are condemned to repeat it. [Santayana] — Borraz

Whats even worse is those who are ignorant of history altogether.The problem is that the greedy capitalists have developed an interest in philosophy in order to control the perception of the masses. This is the only way in which the few can control the many. Capitalism is only rational if idealism is false and materialism is true. But of course, idealism is true, and this, I think, is the reason for the widespread popularity and institutional acceptance of postmodernism, which is shoved down the throats of academics from power structures above them.

Capitalism, as the dominant economic system, is not responsible for all the problems that one has with one’s partner...

Capitalism, as the dominant economic system, is not responsible for all the problems that one has with one’s partner... — Borraz

I'm saying that the problem lies in greed, not in capitalism, for greed is prior to capitalism in the sense that capitalism is an effect of greed and not the other way around. I have no partner, nor any problems with a partner. Not everything is reducible to sex.

I reckon these to be the derivatives of the first principle, which in turn presupposes a third derivative: the goal. The goal in itself might be and optimally is a first principle to any branch of philosophy. — Eleonora

The rest of your post makes me think that you maybe don't mean what this first part sounds like to me, but this first part sounds like something I would agree with. Those two "first principles" I gave derive immediately from the goal of philosophy:

The characteristic activity of philosophy is the pursuit of wisdom, not the possession or exercise thereof. Wisdom, in turn, is not merely some set of correct opinions, but rather the ability to discern the true from the false, the good from the bad; or at least the more true from the less true, the better from the worse; the ability, in short, to discern superior answers from inferior answers to any given question. — The Codex Quaerentis: The Metaphilosophy of Analytic Pragmatism

My general philosophy could be most succinctly summed up as the rejection of both unquestionable answers, and unanswerable questions. By this I mean the commitment to questioning everything, and rejecting anything that's beyond questioning, but also to trusting that there are answers to be had, and entertaining the possibility of anything that might be an answer:

I say to hold that there is some opinion or another that is actually correct in a sense beyond merely someone subjectively agreeing with it, a position that I call "objectivism"; in contrast to its negation that I call "nihilism", by which I mean the view that holds there are no genuinely correct answers.

I say also to hold every opinion open to questioning, a position that I call "criticism"; in contrast to its negation that I call "fideism", by which I mean the view that holds there are some things that are beyond question.

I say to freely hold some tentatively opinion or another on what the answer might be even if you don't have conclusive justification to say that it definitely is that, a position that I call "liberalism"; in contrast to its negation that I call "cynicism", by which I mean the view that holds that no opinion should be held until it can be conclusively justified from the ground up.

And I say to reject any opinion that is not amenable to questioning because it is beyond any possible experience that could test it one way or another, a position that I call "phenomenalism"; in contrast to its negation that I call "transcendentalism", by which I mean the view that holds that there are some things that are utterly beyond the ability to discern from our experiences.

[...]

The underlying reason I hold this general philosophical view, or rather my reason for rejecting the views opposite of it, is my metaphilosophy of analytic pragmatism, taking a practical approach to philosophy and how best to accomplish the task it is aiming to do. As explained above: this view, commensurablism, is just the conjunction of criticism and objectivism, which are in turn just the negations of fideism and nihilism, respectively. If you accept fideism rather than criticism, then if your opinions should happen to be the wrong ones, you will never find out, and you will remain wrong forever; and if you accept nihilism rather than objectivism, then if there is such a thing as the right opinion after all, you will never find it, and you will remain wrong forever. There might not be such a thing as a correct opinion, and if there is, we might not be able to find it. But if we're starting from such a place of complete ignorance that we're not even sure about that — where we don't know what there is to know, or how to know it, or if we can know it at all, or if there is even anything at all to be known — and we want to figure out what the correct opinions are in case such a thing should turn out to be possible, then the safest bet, pragmatically speaking, is to proceed under the assumption that there are such things, and that we can find them, and then try. Maybe ultimately in vain, but that's better than failing just because we never tried in the first place.

[...]

As Henri Poincaré rightly said, "To doubt everything or to believe everything are two equally convenient solutions; both dispense with the necessity of reflection." (La Science et l'Hypothèse, 1901). Or as Alfred Korzybski similarly said, "There are two ways to slide easily through life: to believe everything or to doubt everything; both ways save us from thinking." I would argue that to do otherwise than to try (even if ultimately in vain) to find answers to our questions, to fall prey to either nihilism or fideism, to deny that there are such things as right or wrong opinions about either reality or morality, or to deny that we are able to figure out which is which, is actually not even philosophy at all. The Greek root of the word "philosophy" means "the love of wisdom", but I would argue that any approach substantially different from what I have laid out here as commensurablism would be better called "phobosophy", meaning "the fear of wisdom", for rather than seeking after wisdom, seeking after the ability to discern true from false or good from bad, it avoids it, by saying either that it is unobtainable, as the nihilist does, or that it is unneeded, as the fideist does. Commensurablism could thus be said to be necessitated merely by being practical about the very task that defines philosophy itself. If you're trying to do philosophy at all, to pursue wisdom, the ability to sort out the true from the false and the good from the bad, you end up having to adopt commensurablism, or else just give up on the attempt completely. — The Codex Quaerentis: The Philosophy of Commensurablism

:clap:

Saint Thomas would say that all human conduct is explained in terms of three capital sins: greed, lust, and anger. The police work alike. All crime is due to greed, lust or anger. In both cases, they restrict the human to the most negative in our species, don’t they?

maybe don't mean what this first part sounds like — Pfhorrest

Alright - let me clarify my sense of the goal. The goal across branches of philosophy runs perpendicular to its first principle. As the question is about the universal first principle, it cannot be the goal in itself. Thereby there are derivatives of the goal which in turn are what you described. Since it runs perpendicular to the goal, the universality of the first principle must therefore embrace the potentiality for the dimensionality of all those branches. That is what a principle to seek common ground first does.

Philosophy is about knowing of what we speak. Therefore the ground is the most important. Thus it must be the first principle to seek it.

Not every way of reasoning works this way, but to find those ways is what each individual goal is about. Philosophy itself is about understanding each individual goal. This is the philosophy of the goal.

In both cases, they restrict the human to the most negative in our species, don’t they? — Borraz

The greatest challenge we face with regards to it is finding their place in agreement. Positivity is about embracing negativity. Only together in all right places can we move beyond and into where optimism might take us.

"The goal" (task, function, purpose), as far as I discern it, is ...

... the struggle against stupidity (i.e. FOOLERY, or 'denial of contingency' (e.g. Rosset's "doubling", Becker's "symbolic self", Zapffe/Camus' "absurd", Spinoza's "bondage"), THAT MALADAPTIVELY SELF HARMS AND/OR HARMS OTHERS) - an infinite, yet reflective, task which I propose uniquely belongs to philosophy ... — 180 Proof