The Isavasya or Isa Upanisad is traditionally placed first in collections of
Upanisads. It belongs to the Vajasaneya Samhita of the White Yajurveda. Unlike
the other Upanisads, it is included in the Samhita of the Veda itself (as ch.
40), rather than in a separate section. It takes its name from its first words
(n.3 below). Some editions transpose vv. 9-11 and 12-14 and vv. 15-16 and 17-18.
The Isavasya is very difficult to date. It shows clear links on the one hand
with the verse passages in Brhadaranyaka. IV, and on the other with the middle
period verse Upanisads, particularly the Svetasvatara. I am inclined to favour
the earlier date, since the author of the latter could well have known the Isavasya
and been inspired by it without necessarily being to it in time.
OM. That is full; this is full;
Fullness comes forth from fullness:
When fullness is taken from fullness,