B.G 18.31

यया धर्ममधर्मं च कार्यं चाकार्यमेव च। अयथावत् प्रजानाति बुद्धिः सा पार्थ राजसी ॥३१॥

yayā dharmamadharmaṃ ca kāryaṃ cākāryameva ca। ayathāvat prajānāti buddhiḥ sā pārtha rājasī ॥31॥

[यया (yayā) - by which; धर्मम् (dharmam) - righteousness; अधर्मं (adharmaṃ) - unrighteousness; च (ca) - and; कार्यं (kāryaṃ) - action; च (ca) - and; अकार्य (akārya) - inaction; मेव (meva) - indeed; च (ca) - and; अयथावत् (ayathāvat) - incorrectly; प्रजानाति (prajānāti) - perceives; बुद्धिः (buddhiḥ) - intellect; सा (sā) - that; पार्थ (pārtha) - O son of Pritha; राजसी (rājasī) - passionate;]

The intellect that perceives incorrectly righteousness as unrighteousness and action as inaction, O son of Pritha, is considered being of a passionate (rājasī) nature.

Gīta Bhāshya 18.31

Inconsistency in understanding the reality AS-IS is the mode of passion, while consistently misunderstanding is the mode of darkness.

यथार्थत्वनियमाभावे राजस्याः। अन्यथा तामस्याः भेदाभावात्॥

yathārthatvaniyamābhāve rājasyaḥ। anyathā tāmasyaḥ bhedābhāvāt॥

[यथार्थत्व (yathārthatva) - understanding reality AS-IS; नियम (niyama) - rule; अभावे (abhāve) - absence; राजस्याः (rājasyaḥ) - of the king; अन्यथा (anyathā) - otherwise; तामस्याः (tāmasyaḥ) - of darkness; भेद (bheda) - difference; अभावात् (abhāvāt) - due to absence;]

In the absence of consistency in adhering to the rule on understanding the reality AS-IS, the mode of passion prevails. Otherwise, it becomes indistinguishable from the mode of darkness.