"It was a custom with Apelles, to which he most tenaciously adhered, never to let any day pass, however busy he might be, without exercising himself by tracing some outline or other; a practice which has now passed into a proverb." (Pliny, Natural History, XXXV, 36)
Pliny tells us that Apelles "surpassed all the other painters who either preceded or succeeded him" and painted with a "singular charm of gracefulness". He lived in the 4th Century BC, which means his proverb "nulla dies sine linea", "no day without a line", has been with us for about two and a half millennia. Michael Gilleland (with a hat tip to Laura Gibbs), has found a classic application of the proverb for writers in the autobiography of Anthony Trollope:
It will not, I am sure, be thought that, in making my boast as to the quantity, I have endeavoured to lay claim to any literary excellence. That, in the writing of books, quantity without quality is a vice and a misfortune, has been too manifestly settled to leave a doubt on such a matter. But I do lay claim to whatever merit should be accorded to me for persevering diligence in my profession. And I make the claim, not with a view to my own glory, but for the benefit of those who may read these pages, and when young may intend to follow the same career. Nulla dies sine linea. Let that be their motto. And let their work be to them as is his common work to the common labourer. No gigantic efforts will then be necessary. He need tie no wet towels round his brow, nor sit for thirty hours at his desk without moving,—as men have sat, or said that they have sat. More than nine-tenths of my literary work has been done in the last twenty years, and during twelve of those years I followed another profession. I have never been a slave to this work, giving due time, if not more than due time, to the amusements I have loved. But I have been constant,—and constancy in labour will conquer all difficulties. Gutta cavat lapidem non vi, sed saepe cadendo.
There are many things to notice in this passage. First, he emphasizes that the aim is quality not quantity (Trollope was famously prolific). Second, he insists on a certain "professionalism", which requires a particular "diligence". Third, he suggests that the writer see the work precisely as a work, "as common work to common labourer". Fourth, he dismisses the idea that in order to write you have sit at your desk for ridiculous lengths of time engaged in ridiculous exertions, an idea he also suspects of being a pose (something writers say they do, more than they actually do it). Finally, he emphasizes that one should not be a "slave" to writing, that one should leave time to enjoy the other things in life and that writing can easily be reconciled with other responsibilities, including holding down an entirely unrelated job.
My version of this advice, of course, is: no day without a paragraph. It nicely matches Pliny's gloss on the proverb, which calls for "tracing some outline or other" every day. We can imagine Apelles spending some time every day, no matter how busy he might otherwise have been, drawing some particular thing, rendering it, representing it, depicting it. He was not merely doodling. He was practicing his art. He was putting down on the page what a thing looks like. By a similar token, I recommend spending time every day, no matter how busy you are, articulating something you know. And by this I do not just mean writing whatever comes into to your mind, but stating a knowledge claim (writing a single sentence you know to be true) and supporting it with a paragraph of prose (about six sentences more). This really should be the mission of our universities: to train students to compose paragraphs that articulate knowledge in their chosen field. We must teach them to put down on the page what is true of things. Constancy in this labour will conquer perhaps not all difficulties, but a great number of the most important ones.