Bhagavad Gita Chapter 18, Verse 32
अधर्मं धर्ममिति या मन्यते तमसावृता | सर्वार्थान्विपरीतांश्च बुद्धिः सा पार्थ तामसी ||
adharmaṁ dharmam iti yā manyate tamasāvṛtā sarvārthān viparītāṁś ca buddhiḥ sā pārtha tāmasī
O Arjuna, that intellect which, covered by tamas, considers non-duty as duty and all things as inverted, is tamasic.
Krishna explains tamasic intellect. 'Adharmaṁ dharmam iti yā manyate tamasāvṛtā'—that intellect (yā buddhiḥ) which, covered by tamas (tamasā āvṛtā), considers (manyate) non-duty as duty (adharmam dharmam iti). 'Sarvārthān viparītāṁś ca buddhiḥ sā pārtha tāmasī'—and all things as inverted (sarvārthān viparītān), that intellect (sā buddhiḥ) is tamasic (tāmasī), O Arjuna (pārtha). This is tamasic intellect: completely wrong understanding, covered by ignorance, seeing everything inverted. It considers non-duty as duty, wrong as right, harmful as beneficial. This intellect is driven by tamas (ignorance), which creates complete delusion. Unlike sattvic intellect (clear, discerning) and rajasic intellect (confused but partially aware), tamasic intellect is completely wrong and inverted. This is the lowest kind of intellect: ignorant, deluded, seeing everything backwards.