I know it's a mistake to even bring this up (I'll check back in when this gets to page 24), but aren't you essentially only making an argument for taxation in general, and not necessarily an argument for a progressive tax? If it is indeed a "common social structure", then why should taxation be...uncommon?
I agree that the statement I made is not necessarily an argument for a progressive taxation structure. Such an argument, which I sense you are reluctant to begin, would need additional ideas that, due to the perceived reluctance, I shall refrain from including.