Sat in Hinduism is existence or the real. Sat is the present principle of ‘as,’ the Sanskrit verb “to be”. The feminine noun sati (good wife) and satya (truth) are related forms. Sat is specifically used as ‘being’ in ontology, ‘truth’ in epistemology and ‘good’ in ethics. For Vedanta, all three are ultimately the same.
The term sat plays a key role in Chandogya Upanishad and Brihadaranyaka Upanishad and thus in all subsequent developments within Hinduism, specifically Vedanta, that depend upon them as textual authorities. In Chandogya Upanishad, Uddalaka instructs his son Svetaketu that the fundamental underlying reality is different from the modifications in that reality that produce the manifold names and forms of the universe. Uddalaka teaches his son that the universe in the beginning was ‘Being’ or ‘Existent’, i.e., sat alone, one without a second. From the progressive modifications of the sat were born the plurality of the existent beings of the universe. Sat created or emitted heat, heat created water and water created food. The highest being, sat, entered by means of the self (atman) into the three created entities of heat, water and food, causing them to recombine into the variety of the existent entities of the universe of cause and effect.
For Uddalaka, the individual person is bound into this universe of multiple forms, derived originally and still informed by the one ‘sat’, by actions resulting from desire. As long as the effects of individual action, karma, remain, so must individual existence remain even through death and rebirth. Is there liberation from this? For Uddalaka, liberation from death and rebirth lies in knowledge. Since the entire universe was, in the beginning, sat alone, one without a second, it still is in essence sat alone, one without a second.
In Brihadaranyaka Upanishad I.3.28, Yajnavalkya teaches that the unreal (asat) is death, and the real (sat) is immortality – “From the unreal lead me to the real, from darkness lead me to light, from death lead me to immortality.” Uddalaka’s and Yajnavalkya’s teaching exerted a significant influence on the subsequent tradition of Vedanta and thus on Hinduism. Sat, the existent, one without a second, is Brahman. Brahman permeates the phenomenal world as the Self. The goal of the human person is release from the unreal by means of true knowledge of the real. Sat is true knowledge.
This teaching forms the basis for the theistic teaching of the Bhagavad Gita, which surmounts and supersedes the insight of Upanishads. For example, in II.16, Krishna states – Of the non-existent there is no coming to be; of the existent there is no ceasing to be. The conclusion about these two has been perceived by the seers. In IX.19, Sri Krishna unites being and non-being when he declares – “I am the being (sat) as well as the non-being (asat), O Arjuna. Finally, in XIII.13, Sri Krishna teaches – I will describe that which is to be known and by knowing which life eternal is gained. It is Supreme Brahman which is beginningless and which is said to be neither existent (sat) nor non-existent (asat).
Sat, the existent, identified with Brahman, thus is the primary teaching of all the forms of Vedanta, theistic and atheistic, Advaita, Vishishtadvaita and Dvaita.