At what period these two sastras first assumed a definite form, we are unable to ascertain. Discussions of the nature of those which constitute the subject-matter of the Purva Mima/m/sa must have arisen at a very early period, and the word Mima/m/sa itself together with its derivatives is already employed in the Brahma/n/as to denote the doubts and discussions connected with certain contested points of ritual. The want of a body of definite rules prescribing how to act, i.e. how to perform the various sacrifices in full accordance with the teaching of the Veda, was indeed an urgent one, because it was an altogether practical want, continually pressing itself on the adhvaryus engaged in ritualistic duties. And the task of establishing such rules was moreover a comparatively limited and feasible one; for the members of a certain Vedic sakha or school had to do no more than to digest thoroughly their own brahma/n/a and sa/m/hita, without being under any obligation of reconciling with the teaching of their own books the occasionally conflicting rules implied in the texts of other sakhas. It was assumed that action, as being something which depends on the will and choice of man, admits of alternatives, so that a certain sacrifice may be performed in different ways by members of different Vedic schools, or even by the followers of one and the same sakha.
The Uttara Mima/m/sa-sastra may be supposed to have originated considerably later than the Purva Mima/m/sa. In the first place, the texts with which it is concerned doubtless constitute the latest branch of Vedic literature. And in the second place, the subject-matter of those texts did not call for a systematical treatment with equal urgency, as it was in no way connected with practice; the mental attitude of the authors of the Upanishads, who in their lucubrations on Brahman and the soul aim at nothing less than at definiteness and coherence, may have perpetuated itself through many generations without any great inconvenience resulting therefrom.
But in the long run two causes must have acted with ever-increasing force, to give an impulse to the systematic working up of the teaching of the Upanishads also. The followers of the different Vedic sakhas no doubt recognised already at an early period the truth that, while conflicting statements regarding the details of a sacrifice can be got over by the assumption of a vikalpa, i.e. an optional proceeding, it is not so with regard to such topics as the nature of Brahman, the relation to it of the human soul, the origin of the physical universe, and the like. Concerning them, one opinion only can be the true one, and it therefore becomes absolutely incumbent on those, who look on the whole body of the Upanishads as revealed truth, to demonstrate that their teaching forms a consistent whole free from all contradictions. In addition there supervened the external motive that, while the karmaka/nd/a of the Veda concerned only