But even playful actions, which seem to be done for no end [absque fine fieri], have a due end, to wit, that through them the mind may be in some way relaxed, so as to be afterward more capable for studious work [potentes ad studiosas operationes]: otherwise, were play sought for itself, one would always have to play, which is inappropriate.
Aquinas SCG III.25.9. I'm almost inclined to translate "quod est inconveniens" at the end with the pseudo-cognate "inconvenient," because it would definitely be that, too. Some translations translate "ad studiosas operationes" as "for serious work," but I think the relation to the virtue of studiousness is deliberate, given what Aquinas says elsewhere. We play because, despite being rational creatures, real thinking is hard work; thus the more we do it, the more we need to relax our minds, lest we wind ourselves up so tightly we break. (This is precisely the image Aquinas uses in discussion the virtue of eutrapelia, good playfulness, in fact.) The importance of balancing study and play is very clear in Aquinas: as bodily rest is to physical work, so play is to mental work. In any case, his primary point here is not that but simply that, even though play seems to be aimless, it actually has a very important aim, to which it must be proportioned. If it had no aim, then play would be an end in itself, and there would be no limit to how much it should be done. But it's not difficult to think of limits for play.