Chapter 4: The Nature of Reality : 5.14.
5. Brahman as God or Ishvara-14.
This is the Nature of Reality as appearing to put on all names, all forms and all actions, though these three aspects are the one being, the Self (Brih. Up., I. 6). The Upanishads do not make much practical difference between Ishvara and Brahman, and hold that “Brahman is both the Formed and the Formless” (Brih. Up., II. 3. 1). They voice both the phenomenal and the absolute points of view.
The proofs for the existence of Ishvara really turn to be proofs for the existence of Brahman. In fact there cannot be any strictly logical proof for the existence of an Ishvara who is different from Brahman. The moment we admit something which distinguishes Ishvara from Brahman, we bring forward a reality which is neither Ishvara nor Brahman.
The Absolute which is ever consistent with itself does not allow in any extraneous principle which would limit Pure Existence. Ishvara is Brahman defined by the creative will. Brahman appears as the supreme Person (Purusha-vidha), and in becoming this it would appear to cease to be what it is, at least temporarily. Such a conception of Brahman would go against the very grain of the reality of Brahman.
To be continued ..