Bhagavad Gita Chapter 18, Verse 22

यत्तु कृत्स्नवदेकस्मिन्कार्ये सक्तमहैतुकम् | अतत्त्वार्थवदल्पं च तत्तामसमुदाहृतम् ||

yat tu kṛtsnavad ekasmin kārye saktam ahaitukam atattvārthavad alpam ca tat tāmasam udāhṛtam

That knowledge which is attached to one action as if it were the whole, without reason, without understanding the truth, and trivial, is declared to be tamasic.

Krishna explains tamasic knowledge. 'Yat tu kṛtsnavad ekasmin kārye saktam ahaitukam'—that knowledge which is attached (saktam) to one action (ekasmin kārye) as if it were the whole (kṛtsnavat), without reason (ahaitukam). 'Atattvārthavad alpam ca tat tāmasam udāhṛtam'—without understanding the truth (atattvārthavat), and trivial (alpam), that (tat) is declared (udāhṛtam) to be tamasic (tāmasam). This is tamasic knowledge: seeing one small thing as if it were everything, attached to it without reason, without understanding the larger truth. It's narrow, trivial, and ignorant. This is the lowest kind of knowledge: not seeing the whole picture, attached to one small aspect, driven by tamas (ignorance). Unlike sattvic knowledge (which sees unity) and rajasic knowledge (which sees diversity), tamasic knowledge sees only one small thing and thinks it's everything.