Ramana MaharshiWhatever thing [in body and mind] acts in whatever way, remain aloof from those activities [that is, from the notion that it is ‘I’ who acts] and simply be a witness to them.

Ramana Maharshi

Tattva Bodha
Knowledge of Reality, by Śaṅkarāchārya

Tattva Bodha Reference Materials:

Video/Audio Lectures

Swami Paramarthananda
36 classes (~30 - 55 mins each)

Swami Anubhavananda
7 classes (~75 mins each)

Tattvabodha English 2 of 7 @ Prem Puri 2012

Swami Sarveshananda
27 classes (~45 - 60 mins each)

Swami Swatmananda
22 classes (~60 - 90 mins each)

20 vedanta fundas- full session with meditation

Swami Narasimhananda
12 classes (~60 - 90 mins each)

Introduction to Vedanta Tattvabodha Narasimhananda 2

Swami Viditatmananda
9 classes (search for "Tattvabodha" on the linked page)