"Study as if you were to live forever. Live as if you were to die tomorrow."
St. Isidore of Seville
That's a great general rule, though it's application is difficult. I can't study and live at the same time. More than once I've asked myself why I'm reading "X and Z" when I could be enjoying the company of others, and I never have a wholly satisfactory answer.
It's interesting, though: Isidore doesn't tell us to work, which is pretty much all I do anymore (so it seems). But I know work is good for the soul, as long as it's not motivated by a kind of money-obsessiveness that is tantamount to selling your soul to the devil. I'm not sure I'm safe on that ground, either, but that's another story/confession.
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By the way: I'm occasionally asked where I get these quotes. The answer is: It varies. Some I find online, some are from in-depth reading, and some are from quote books. But my number one source (and my source for today's quote) is Mike Aquilina's excellent The Way of the Fathers.