I was called by an academic at the Judge Institute at Cambridge University late last week. They were looking for a recent theoretical justification for progressive taxation. It's some indication of the current state of thinking that I am aware of the lack of this, and have been in discussion with some others about the need to create one for some time. So I could not help.
If the ultimate objective of politics is to make us autonomous active citizens, then the state must deliver the capabilities necessary for people to achieve such self-empowerment. Of course education matters - not just to learn to earn but for the rest of life, beyond work. If people are to be genuinely free they must be sufficiently equal. This requires the state to redistribute resources and raise taxes to pay for everyone to be free.
It's not perfect, and it's not academic, but I like it a lot.