From Imperial Spain: 1469-1716, by J. H. Elliott (Penguin, 2002), 2nd ed., Kindle Loc. 528-549:
The Catalan Diputació was … an immensely powerful institution, backed by large financial resources; and its obvious attractions as a bulwark of national liberty had stimulated Aragonese and Valencians to establish similar institutions in their own countries by the early fifteenth century. As a result, all three states were exceptionally well protected at the end of the Middle Ages from encroachments by the Crown. In the Diputació was symbolized that mutual relationship between the King and a strong, free people so movingly expressed in the words of Martin of Aragon to the Catalan Corts of 1406: ‘What people is there in the world enjoying as many freedoms and exemptions as you; and what people so generous?’ The same concept was more astringently summarized in the famous Aragonese oath of allegiance to the king: ‘We who are as good as you swear to you who are no better than we, to accept you as our king and sovereign lord, provided you observe all our liberties and laws; but if not, not.’ Both phrases, one emotionally, one legalistically, implied that sense of mutual compact which was the foundation of the Catalan-Aragonese constitutional system.
It was typical of the medieval Catalans that their pride in their constitutional achievements should naturally prompt them to export their institutional forms to any territories they acquired. Both Sardinia (its conquest begun in 1323) and Sicily (which had offered the Crown to Peter III of Aragon in 1282) possessed their own parliaments, which borrowed extensively from the Catalan-Aragonese model. Consequently, the medieval empire of the Crown of Aragon was far from being an authoritarian empire, ruled with an iron hand from Barcelona. On the contrary, it was a loose federation of territories, each with its own laws and institutions, and each voting independently the subsidies requested by its king. In this confederation of semi-autonomous provinces, monarchical authority was represented by a figure who was to play a vital part in the life of the future Spanish Empire. This figure was the viceroy, who had made his first appearance in the Catalan Duchy of Athens in the fourteenth century, when the duke appointed as his representative a vicarius generalis or viceregens. The viceroyalty – an office which was often, but not invariably, limited to tenures of three years – proved to be a brilliant solution to one of the most difficult problems created by the Catalan-Aragonese constitutional system: the problem of royal absenteeism. Since each part of the federation survived as an independent unit, and the King could only be present in one of these units at a given time, he would appoint in Majorca or Sardinia or Sicily a personal substitute or alter ego, who as viceroy would at once carry out his orders and preside over the country’s government. In this way the territories of the federation were loosely held together, and their contacts with the ruling house of Aragon preserved.