The Western Catholic intellectual tradition is built as it were in layers of texts, with one writer commenting upon another, and then further writers commenting upon that one. Consider, for example, the following chain of commentaries: the Neo-Platonic philosopher Porphyry wrote his Isagoge, which was a commentary on and introduction to parts of Aristotle's logical texts (his Organon); St. Boethius, in turn, wrote a commentary on the Isagoge, as well as on parts of Aristotle's Organon e.g. the Categories and the De interpretatione. Peter Abelard also wrote a commentary on the Isagoge as well as on various parts of the Organon, but his Isagoge commentary is in large part a commentary on Boethius' commentary on the Isagoge. Abelard in turn influenced later commentators on these logical works.
To "comment" is to engage in a paradigmatically traditionary actitivty. It is not merely to merely explain the meaning of the text on which one is commenting. Rather, it is also to build upon that text, to allow that text to prompt questions that must be answered, to apply that text to one's context, but in such a way that the concerns of the original text are in some ways normative for the questions one raises. It is to recognize that certain texts are indeed authoritative, certain questions that have been raised by the tradition before one must be answered anew, that one's style and order of thought must be received not invented. Commenting requires creativity entirely embedded in docility and humility.
In the twelfth century, Peter Lombard, master at the Cathedral School of Notre Dame in Paris, compiled his Sentences. This was a compilation of texts from Scripture and the Fathers on all topics in theology, arranged topically in four books (on the Trinity, Creation, the Incarnation, and signs e.g. sacraments). This textbook became the normative text for students of Catholic theology at least until the seventeenth century. All of the major thinkers of the scholastic tradition commented and raised questions on it, or on of its successors. Aquinas, Bonaventure, Scotus, Ockham, and Capreolus all commented on the Sentences; later thinkers commented, for example, on Scotus' various commentaries on the Sentences, or on Aquinas' Summa theologiae, itself a development of his commentary on the Sentences. Through this text (as well as through the texts of other normative thinkers e.g. Aristotle, Avicenna, Dionysius, the Bible, the Decretals, etc.) the Western tradition was tied together so as to have a common basis. To read in the scholastic tradition is to read a great conversation, bound together by a common love for these texts, by a particular order of thinking and inquiry, by docility to the received tradition, and, within that docility great flexibility and inventiveness of thought (which qualities can only really thrive if one has a firm foundation).
We don't have anything like the tradition of commenting on the Sentences now. In the scholastic revival of the nineteenth and twentieth century, some thinkers (e.g. the great Thomist Reginald Garrigou-Lagrange) wrote commentaries on the Summa theologiae, and many important manuals were written, following the classic order of inquiry rooted ultimately in the Sentences. And later thinkers following to some extent the scholastic tradition (e.g. the current burgeoning field of analytic theology) have not engaged with the tradition via the classic commentary genre (though there are some things that are kind of like it e.g. Robert Pasnau's Thomas Aquinas on Human Nature). Instead, we write journal articles or books, which draw upon the tradition, but raise primarily either exegetical concerns or concerns of contemporary relevance.
I should like to see a resurgence of the tradition of commenting on the Sentences--that is, a resurgence of texts that build explicitly upon Lombard's text, that primarily use the disputed question format of writing (as opposed to the essay) with its dialogical structure of objections and replies, and that both raise and deal with the classic questions asked in these commentaries, and extend this tradition to contemporary concerns, while engaging both with the classic authorities of the scholastic tradition and engaging with modern and contemporary, non-scholastic authors.
To genuinely think with the Catholic tradition requires a rigorous formation of one's thinking, a submergence in the stream of that tradition. I often underestimate the dangers of not so submerging my thinking. It is very easy, at least for me, to adopt habits of mind that are implicitly secular, modernist, naturalistic, anthropocentric; as a result, it is easy to begin to doubt core teachings of the Church e.g. the reality and eternality of hell or the supernatural causal efficacy of the sacraments. It is necessary, if one is to think well, to combat these tendencies by a constant sacrifice of oneself to what is given in the tradition, to make it the air one breathes. This must be done first and foremost, I think, in one's liturgical and devotional life, by a conscious submission to the traditional liturgy and devotions passed down to us, to have a fertile soil in which to grow, wherein one is constantly oriented first and foremost to God, not to oneself or to the human person. But this submission must be done intellectually too. And this is hard to do nowadays, since one is not professionally formed to do this, but rather to write on small, specific topics, in non-traditionary genres. The Sentences, by contrast, encourage one to develop one's thinking in the full breadth and scope of topics in the Catholic tradition, without any sacrifice of depth.
If we were to revive the tradition of commenting on the Sentences, this would go a long way, I think, to encouraging this intellectual discipline. It would root one in the classic words, texts, order of inquiry, and style of questioning of the Catholic intellectual tradition, and give one a basis for integrating the truths found in "post-Christian" thinking back into the Catholic intellectual tradition (much as Aquinas and others integrated the post-Christian thinking of e.g. Porphyry, Proclus, and Averroes into the Catholic tradition). It would give to the world a distinctly Catholic philosophical style and vocabulary--which, I think, only realist scholasticism has the resources to do. (Much as I think they provide important helps to the Catholic intellectual tradition, phenomenology is just insufficiently realist and systematic, and analytic philosophy is also not realist enough e.g. on existence itself--but these approaches certainly should aid the Catholic thinker nowadays.)
Why comment on the Sentences and not e.g. the Summa theologiae? I think we need to return to a text that provided a common basis and medium for all the various schools of orthodox Catholic thinking (Thomism, Scotism, Bonaventurian thinking, the various Jesuit schools, etc.) The new commentator on the Sentences should certainly avail himself of all the resources in later works by scholastic authors (and by many other sorts of authors too). But I think the basis needs to be something more common--not because I think older is ipso facto better or purer, but because of the place the Sentences have in Western Catholic thinking as both Patristic and systematic. (If there were a text that had this place as a common, systematic root of both Western and Eastern thinking, that would be even better, but I don't think there's anything that quite fits the bill.)
I really think that a new tradition of commenting on the Sentences would be a great blessing for Catholic thinking, a new Springtime, if you will. Tradition-minded people have recently made great strides (many obstacles notwithstanding) in recovering a way of life rooted in traditional liturgy and politics, for example, and there have been many initiatives for intellectual thinking as well--but I do think they need something like this to tie them all together. I envision vast commentaries, full of subtle and beautiful reasoning (the more distinctions the better!), some written singly but others written by teams (like the great commentary projects of the seventeenth century at Salamanca, Alcala, and Coimbra, for example), all for the glory of God and the salvation of souls.
I'd be interested to hear from my fellow Catholic thinkers about what they think about reviving this sort of tradition, or if there are already projects like this underway.