There was practically no systematic theory of causation in the Upanishads. Shankara, the later exponent of Vedanta philosophy, always tried to show that the Upanishads looked upon the cause as mere ground of change which though unchanged in itself in reality had only an appearance of suffering change.

This he did on the strength of a series of examples in the Chandogya Upanishad (VI. 1) in which the material cause, e.g. the clay, is spoken of as the only reality in all its transformations as the pot, the jug or the plate. It is said that though there are so many diversities of appearance that one is called the plate, the other the pot, and the other the jug, yet these are only empty distinctions of name and form, for the only thing real in them is the earth which in its essence remains ever the same whether you call it the pot, plate, or Jug. So it is that the ultimate cause, the unchangeable Brahman, remains ever constant, though it may appear to suffer change as the manifold world outside. This world is thus only an unsubstantial appearance, a mirage imposed upon Brahman, the real par excellence.

It seems however that though such a view may be regarded as having been expounded in the Upanishads in an imperfect manner, there is also side by side the other view which looks upon the effect as the product of a real change wrought in the cause itself through the action and combination of the elements of diversity in it. Thus when the different objects of nature have been spoken of in one place as the product of the combination of the three elements fire, water and earth, the effect signifies a real change produced by their compounding. This is in germ (as we shall see hereafter) the Parinama theory of causation advocated by the Samkhya school (1).
1: Cha. VI. 2-4.

brahman, causation, chandogya upanishad, fire water, material cause, samkhya, systematic theory, upanishads