Why 'play always' rather than study always? Our bodily constitution unfits us to do either one or the other. But what St Thomas means is that, if play were an end in itself, we should play all we can, which sundry men do, -- and boys, if you let them. The passage is suggested by Aristotle, Eth. Nic. X, vi, 6. St Thomas does not observe the delight of the artist in the work of his hands, nor consider how few scholars study for study's sake, nor generally how much of pure speculative interest is blended with practice, and how much thinking is done for material gains. As Aristotle might have said, to de kai theôrountes dêmiourgoumen kai pros lêmmata theôroumen.
Of God and His Creatures: 3.25