Bhagavad Gita
18.32

अधर्मं धर्ममिति या मन्यते तमसावृता | सर्वार्थान्विपरीतांश्च बुद्धिः सा पार्थ तामसी ||

adharmaṁ dharmam iti yā manyate tamasāvṛtā sarvārthān viparītāṁś ca buddhiḥ sā pārtha tāmasī

Translation

That intellect enveloped in darkness which regards adharma as dharma and sees all things perverted — O Arjuna, that is tamasic intellect.

Interpretation

Tamasic intellect: completely enveloped in darkness (tamasaavrita — shrouded, suffocated by tamas), this intellect calls adharma 'dharma' — it inverts all values. What is right appears wrong; what is wrong appears right; all things are seen in reverse (sarva-arthan viparitam). This is not mere error but systematic inversion of values — the state where someone is not confused about the good but actively believes that harm is good and good is harmful. History's most destructive ideologies have been sustained by this tamasic intellect.