A commonplace book is a book in which a reader, over his lifetime, and especially in his student years, writes down excerpts from his reading which strike him as so important, and so well expressed, that he wants to treasure them and live by them.

It should be a physical book, not a computer file.  He should insist on a high standard for inclusion, such that there is no dross: everything in it seems to be wisdom.  For example, among the first entries in my own commonplace book from my student years were sentences from Newman:

“In the present day mistiness is the mother of wisdom.”

From Thomas à Kempis:

“I would rather feel contrition than know how to define it.”

And even from Thomas Hobbes:

“When Reason is against a Man, a Man will be against Reason.”

You can see immediately that a commonplace book will not be a book in which someone writes what we call “commonplaces.”  We use the word “commonplace” to mean something obvious, ordinary, and overused.  But originally the term meant simply a theme (Gr. topos, L. locus) which was of value to everyone generally (Gr. koinos, L. communis).

A commonplace book, then, is a book in which you write down – when you discover it, as if you came upon treasure buried in a field! – an expression or passage that is not appreciated to be, but which you think ought to be, words which every wise person might live by.

“Commonplacing” in this sense is not the same as journaling or note-taking, for a class or for some definite research project.  It is a preeminently liberal practice.  John Locke wrote a short manual on commonplacing; Oxford and Harvard used to give courses on “how to commonplace.”  It should be no surprise that the practice fell into desuetude; but it should be a surprise that Catholic educators have so far, it seems, not aimed to revive it, just as they aim to convey good study habits to students.

It’s never too late to start a commonplace book.  Purchase a sturdy notebook with at least a hundred pages and set a goal of writing down perhaps one passage a week.  The Church offers you a quick way to begin populating your book, if you wish to follow it.  I mean that the Catechism of the Catholic Church can be understood to be the Church’s own commonplace book, for us to draw upon, here and now.

Particularly in the Catechism’s widespread use of quotations drawn from the Fathers and from saints.  It used to be the case, I am told, that seminarians in doctrine classes, although taught from manuals, would keep on their desk two books, representing Scripture and Tradition: the Vulgate, and Denzinger’s Enchiridion.  (The latter is a famous compendium of Church teachings in Greek and Latin, abridged and translated in The Church Teaches.)

The Catechism of the Catholic Church in its footnotes, likewise, references Holy Scripture and Church documents pervasively.  But an innovation of that Catechism, much celebrated and praised when it was first released, are the many quotations interwoven into the catechesis from patristic and hagiographic sources.

A Catholic could compose an excellent commonplace book simply by scanning the footnotes and picking out those quotations that struck him as especially valuable.  Fittingly, the opening lines of St. Augustine’s Confessions are the first quotation from the Fathers (n. 30):

You are great, O Lord, and greatly to be praised: great is your power and your wisdom is without measure, and man, so small a part of your creation, wants to praise you. . . . You yourself encourage him to delight in your praise, for you have made us for yourself, and our heart is restless until it rests in you.

You certainly know the end of that passage, but you probably do not know or remember that the beginning dealt with praise.

Likewise, the very last such quotation in the Catechism (n. 2856) is from St. Cyril of Jerusalem, about how the Lord’s Prayer is to be concluded:

Then, after the prayer is over you say “Amen,” which means “So be it,” thus ratifying with our “Amen” what is contained in the prayer that God has taught us.

Granted, St. Cyril’s remark on its own is not especially memorable. Yet someone might wish to write it in his commonplace book, say, as a reminder to avoid the widespread but faulty American practice of omitting the “Amen” after the Our Father in the Rosary.

In the Catechism’s immensely practical passages on marriage one finds, beside the astounding famous and beautiful passage from Tertullian (n. 1642 – I’ll leave that for you to look it up), also this gem from St. John Chrysostom (n. 1620):

Whoever denigrates marriage also diminishes the glory of virginity. Whoever praises it makes virginity more admirable and resplendent. What appears good only in comparison with evil would not be truly good. The most excellent good is something even better than what is admitted to be good.

Likewise in its teaching on prayer one finds:

God wills that our desire should be exercised in prayer, that we may be able to receive what he is prepared to give. (St. Augustine, n. 2727)

It is possible to offer fervent prayer even while walking in public or strolling alone, or seated in your shop. . .while buying or selling. . .or even while cooking. (St. John Chrysostom, n. 2743)

Nothing is equal to prayer; for what is impossible it makes possible, what is difficult, easy. . . .For it is impossible, utterly impossible, for the man who prays eagerly and invokes God ceaselessly ever to sin. (St. John Chrysostom, n. 2744)

Those who pray are certainly saved; those who do not pray are certainly damned. (St. Alphonsus Liguori, ibid.)

Commonplacing shows that what economists call “crowding out” can work for the good as well as the bad.  Why not crowd out snarky tweets (“posts”), and the like, by commonplacing from the saints?