So here is the absurd consequence of your belief/assumption:
"Second, science's purpose is to study that external world, explore, describe and seek to
explain it. So the belief that it exists is built into the very nature of science (and I think into the evolved nature of humans, indeed all animals)."
The word "explain" is not external to the self
It may not be external to the author's self at the time of writing, but if I'm not the author then the explanation is something my self obtains from the external world.
So here is the problem again: You understand what I have written. That is in your self/mind in the internal world. You now choose to answer, but that involves your computer, which is in the external world.
Just so ─ self takes the message in, composes the message out, manipulates the computer and creates an external record of my response. And this external record goes to you, your self reads and understands it, composes the message out, and so on. The same thing is done in conversation, where instead of a screen we have vibrations of the external air.
So here it is for philosophy and your attempt of being authoritative for all humans:
So you claim that you speak with authority over all human reason. You don't!!! You are in effect an authoritarian, who claim power over reason. You don't have that and you should really stop doing that. You are in effect no different than some dogmatic, fundamentalist religious people. You believe you are the correct source of a "we". You are not!!!
The world is not an external world. The world is also you with your self and you are going to prove that, because you are going to post your internal understanding to counter mine. But that is the proof. That you are a self in the world and a part of it and that you communicate with another self, me.
So that is the limit of your philosophy:
You rely on words, which are not true, because they have no correspondence to the external word and that is the only truth, you accept. But that is not true, because that you accept it, is in the internal world. So you start your system by using non-truth. The world is in part false, because all the internal parts of it are false and that includes you as a self. You are false.
Yet you are not, right?!!! Of course, you are not false, wrong or any of those. You are not even irrational. You are just not aware of the limitations of this:
- that reason is a valid tool.
So here it is for human mobility. Human mobility is a part of the world, but it has limits. The same is the case with reason and your idea that it has to be valid. I am a skeptic and to me reason is a human behaviour in the self, that has limits. Just like truth and logic.
And you are in effect apparently incapable of doubting the limits of your assumptions of how they work in practice. In effect you do the following trick in your thinking. For "that reason is a valid tool" it is a case of A is B in time, space and at least one case. But you are unable to test if there is for another case in another time and space, where reason is not a valid tool.
So here it is for the limits of your assumptions and your kind of science as above:
For the everyday world you can't with your reason, correspondence truth and science do these human behaviours:
https://undsci.berkeley.edu/article/0_0_0/whatisscience_12
You can't do good and useful, and you assume that your good and useful as it is in your post, is universal for all humans. It is not and it will never be that. You are like all other humans a product of nature and nurture and you take your understanding for granted as the correct one for all humans. It is not so.
I don't have to accept your cognition as for your 3 assumptions, because I can doubt them and replace them with another philosophical system.
The world is how it makes sense to humans. That involves at least 3 interconnected categories, which can't be reduced to less than these 3:
The objective as physical.
The inter-subjective as social.
The subjective as individuality.
You try to reduce the world down to the first one and I just do the social and individual differently and that is the falsification of your individual belief in: - that reason is a valid tool.
Reason is a useful, bit limited human behaviour. And so is your belief in objective correspondence truth. I just have to do it differently thus I falsify your "we".
So here it is reduction as absurdum. Since I don't use reason as a valid tool like you, I have already walked out in front of an oncoming truck and is longtime dead. I am so wrong, because I different than you that I am not even in the world as different than you and I haven't written all of this, because I am not like you. So I am not at all!!! I am not in reality and the world. I am so irrational and without your objective, universal, true reason, that you are not even reading this.
Start being a skeptic and doubt yourself!