You make assessments based on everything else you know (I think he mentions that at one point).
Of course this isn't unbiased, but at least you're thinking for yourself.
I don't think it's just about thinking from first principles, it's also about being able to change your first principles as you go along and understand that you need to reassess everything else that depends on it.
In the end he says that after having had 5 kids he believes that nature has a much stronger impact on people's personalities than nurture. That surprised me, I wish he could have elaborated on it.
Of course this isn't unbiased, but at least you're thinking for yourself.
I don't think it's just about thinking from first principles, it's also about being able to change your first principles as you go along and understand that you need to reassess everything else that depends on it.