(Sanskrit = Sandilya's Knowledge)
Sandilya's statement that "Atman is brahman", i.e. all of us contain an essence or soul that is part of Brahman, the hidden spiritual driving force of the universe.
In Sanskit the knowledge, itself, is called Tat tvan asi, the Eternal is in you.
1. "In the Satapathabrahmama (X.6.3) in a passage on meditating on brahman, he [Sandilya] is represented as saying that one should meditate on the atman, soul or self ….
This probably means that the brahman, the absolute, ultimate or primaeval reality, is the same as the individual soul.
In any case in the Chandogya Upanisad (111.14), the latter doctrine is stated clearly and attributed to Sandilya …. Here he says 'my soul ( atman) is brahman' (111.14.4), …. Again there is a similar, but not identical, wording in the Brahmana version. This equation of atman and brahman was subsequently known as the Sandilya Vidya, Sandilya's Knowledge."
(From A Course in Indian Philosophy By Anthony Kennedy Warder
2. Click here to read a summary of the Chandogya Upanishad.