Amongst Rishis, there are some who consider the study of the Vedas to be a some sacrifice; and who consider contemplation to be a great sacrifice which they perform in their minds.
स्वाध्याययज्ञा ऋषयो ज्ञानयज्ञास्तथा परे। अथापरे महायज्ञान् मनस्येव वितन्वते॥