Here's Jonathan on the aforementioned latin bit. (& he becomes reader # 8 of Stubborn Grew, the book that changed history (at least until the Johnson-Silliman Pact of 2004, outlined by that Walden guy) - thanks - & congratulations!).
I would never think of Robert Creeley in relation to that notion. Ars et celare artem makes me think, not so much of plainness & simplicity, as of a kind of crafty guile, combined with all sorts of indirections, including perhaps a surface moderation or transparency. But Jonathan's concept may be more accurate.
This gets me to Jordan's aside on "depth". I'll have to think a little more deeply about this. Somewhere in the book Hamlet's Mill, mentioned here a while back, there is something about archaic astronomy-astrology as the search for. . . man, now I can't think of the word! It's not consistency. . . it's not continuity. . . but it's like that. . . it's like a philosophical term for steady duration, a pattern which remains the same. . .
Anyway, what I'm trying to say is that it seems to me that hermeneutics or interpretation of art works often involves searching for the deep motive, the wellspring which connects all the dots, the answer to why an artist or writer chose this particular obscure symbol or image or emphasis.
The presence of such a motivating plot or plan is one signature of depth. You could also talk about an artwork's "emotional depth", but such is usually brought, paradoxically, to the surface (the crisis, the denouement). Depth of purpose, on the other hand, often remains hidden & disguised - and the reason for this is because there is the deepest emotional commitment to the deep-ocean argument which actually triggered the work in the first place.