S Prosper of Aquitaine was so close a collaborator with S Leo the Great that it is not always entirely clear which of S Leo's writings are his, and which might owe much to S Prosper. It was he who formulated the adage that "the Law of Praying should establish the Law of Believing" (obsecrationum quoque sacerdotalium sacramenta respiciamus, quae ab apostolis tradita in toto mundo atque in omni catholica ecclesia uniformiter celebrantur, ut legem credendi lex statuat supplicandi.) It is, in other words, to the Law of the Church's Prayer that we should turn in order to find sound doctrine.
I am a trifle uneasy about the way in which Pius XII flirted (Mediator Dei, 1947) with the notion of reversing this formula so as to make the Law of Believing establish the Law of Praying, and of retaining both versions in, as it were, creative tension. This could easily relegate the Law of Praying to the humble status of a resource occasionally plundered by those drafting pontifical documents and needing proof-texts to bolster up a weak argument with impressive footnotes. This demotion of the Law of Praying seems indeed to be precisely what happened in the disordered years after the Council, when the levers of liturgical power in the Church fell into the hands of a tendenz determined to make their own presuppositions the dominant norm according to which liturgical texts would be judged and changed or even (as in the rite for the Consecration of bishops and by the provision of "Alternative Eucharistic Prayers") dumped.
An expression of this desire to make the Lex credendi determine the Lex orandi (rather than the correct way round) can be found in the Decree of 1951 ordering a new Office for August 15: " ... congruum erat ut etiam Officium iis adornatum esset laudibus, quae Deiparae Virgini ob definitum corporeae Assumptionis dogma merito tribuendae erant".
As so often, when one really looks into matters, Pius XII turns out to be the real progenitor of the "Post-Conciliar Reforms". Hannibal Bugnini forsaw this very clearly when he wrote in 1956 that the pope who had been, first, the Restorer of the Vigil and then the Restorer of Holy Week, would become the Supreme Restorer of the entire sacred Liturgy (totius sacrae Liturgiae Summus Instaurator).
We did not then and do not now need a Supreme Restorer of the entire Liturgy. We need a recovery of respect for the ancient liturgical texts so that they can shape the beliefs of the worshipping community.
I don't need anybody to lecture me about the shortcomings of the Novus Ordo. But, viewed incrementally, the first step in true liturgical restoration might be, not so much the immediate prohibition of the entire Novus Ordo as the immediate outlawing of those Cuckoo's Eggs in the Roman Liturgy: the unRoman and orientalising "Alternative Eucharistic Prayers".