The Latin phrase in today’s comic is cited from John Adams quoting Horace’s Ars Poetica in a letter to JQA in 1777. Note the date as this would have been addressed to a nine-year-old. Yes, “modern-day lens” and all that but even for the time, as bright as these two men were, they were very strict and dull as fathers.
Here’s the letter below. Get ready for fun, as according to John Adams, learning is the best entertainment:
John Adams to John Quincy Adams:

Philadelphia March 16. 1777

My dear Son

There is an observation, which I wish you to make very early in Life because it may be usefull to you, when you grow up. It is this, that a Taste for Literature and a Turn for Business, united in the same Person, never fails to make a great Man. A Taste for Literature, includes the Love of Science and the fine Arts. A Turn for Business, comprehends Industry and Application as well as a faculty of conversing with Men, and managing Affairs.

I hope you will keep these two Objects in View all your Lifetime. As you will not have Property to enable you to pursue your Learning alone, you must apply yourself to Business to procure you the Means of subsistance. But you will find Learning of the utmost Importance to you in Business, as well as the most ingenious and elegant Entertainment, of your Life.

You must acquire the Art of mixing Study with Business and even with your Pleasures and Diversions.—Omne tulit punctum, qui miscuit utile dulci.—There is a Bone for you to pick. Ask Mr. Thaxter, after giving my Love to him, to help you Parse this latin Line. Take it Word by Word, and be able when I come home to give me the Construction of the Line, and the Parsing of every Word in it.

—I am your affectionate Father.



↓ Transcript
Panel 1:
Abigail Adams shouts off-panel: "John Quincy Adams, come say hello."
John Quincy Adams runs in with a "zip!"
John Quincy Adams: "Father, as you wanted, I've been studying Latin instead of being a normal kid."
John Adams: "Ah good! Omne tulit punctum, qui miscuit utile dulci."

Panel 2:
John Quincy Adams: "I know that phrase now! 'He gains everyone's approval who mixes the pleasant with the useful.'"
John Adams: "You'll find being able to translate Latin to be very useful."

Panel 3:
John Quincy Adams: "Speaking of mixing the pleasant, can I go have fun now?"
John Adams: "You're in luck! They wrote latin phrases for centuries."