When, in 1950, Papa Pacelli defined the dogma of the Corporeal Assumption of the Mother of God, the formula with which he did so very carefully avoided saying either that she died before her Assumption, or that she did not die (expletu terrestris vitae cursu). This definition had the practical effect of eliminating from the devotional life of Catholics much of the 'apocryphal' narrative which, in both the East and the West, had surrounded the Eschaton of the Theotokos. Prayers which are found in earlier Western liturgies (e.g. festivitas ...in qua dei genetrix mortem subiit temporalem ...) became unusable; many iconographic representations became problematic; tropes, such as that of S Gregory Palamas, explaining to prepon that she had to die to be like her Son, while by no means excluded as pious opinions, became beliefs which it was impossible to describe as the Teaching of the Church. In effect, far from being a novel imposition, the doctrine proclaimed in 1950 constituted the elimination of 95% of what had previously been taught or believed. What was left was but an austere and minimalist doctrinal skeleton of the rich narrative tapestries which nourished Christians from Ireland to India before the Definition.
The root within the verb/noun definire/definitio is -fin-, meaning a boundary. To define a proposition is thus to place boundaries round it, to limit it. While, therefore, a definition may make an additional claim upon the consciences of some, upon others it is likely to have the quite opposite effect. Foliage surrounding the defined doctrinal core has, in effect, been scythed away.
In 1870, the Decree Pastor aeternus imposed an additional claim upon 'Gallicans' and 'Conciliarists': they were obliged to believe that the Roman Pontiff was infallible. But he was only described as infallible in matters of Faith and Morals. That is limiting. Neque enim Petri successoribus Sanctus Spiritus promissus est ut eo revelante novam doctrinam patefacerent, sed ut, eo assistente, traditam per Apostolos revelationem seu fidei depositum sancte custodirent et fideliter exponerent.
How does this bear upon the vexing question of the exact binding force of a canonisation?
Theologians had for centuries discussed the possession by the Roman Bishop of an infallible magisterium. But they had not conducted that discussion within the tight boundaries of the 1870 Definition. If a theologian writing BEFORE 1870 asserted that X had been infallibly taught, you cannot fairly claim that he asserted X to have been infallibly taught in THAT sense of Infallibility which was defined in 1870. He may be thinking in broader, or narrower, categories than those of Pastor aeternus.
Thus, when writers of the eighteenth or earlier centuries argue that Canonisations are infallible, they are not claiming that a canonisation concerns Faith or Morals and that it is part of the Revelation handed on by the Apostles ... for rather obvious reasons: if the Saint lived in the sixteenth century, their sanctity can clearly not be part of that immutable body of truth which was taught and believed also in the fifth and fifteenth centuries; and Saint So-and-so did not exist within the depositum which the Apostles tradiderunt.
I share the view of Benedict XIV, writing as a private doctor, that questioning a canonisation is temerarious. Nor do I deny the propriety of any use of the I-word with regard to canonisations. But it seems to me clear that a canonisation cannot claim that infallibility, that binding force, which the Decree Pastor aeternus of 1870 attributes to the Roman Pontiff when speaking ex cathedra.
I have returned to this question because the current, apparently politically motivated, frenzy for canonising recent Bishops of Rome will not, I very much fear, be sated until Paul VI has been given his gong too. And his case may be more problematic for decent faithful Catholics than that of S John XXIII or S John Paul II. I think we had better get our thinking straight before it happens, so that we know what we are going to say that it means. That means addressing the question I have touched upon here and which Ad tuendam fidem touches (i.e. with what precise degree of authority is a canonisation proposed to us); as well as the next question: what precisely, not according to opinions but in terms of the express words of the Magisterium, does a canonisation expect us to believe with regard to the one canonised?