Bhagavad Gita, Chapter 18, Verse 41

Of Brâhmanas and Kshatriyas and Vaishyas, as also of Sudras, O scorcher of foes, the duties are distributed according to the Gunas born of their own nature.

Real duty here is tied to actual inner quality, not to birth or circumstance.

Krishna grounds this social structure in something real and internal, the guṇas, rather than something external and fixed. That distinction matters for how the whole framework should actually be understood.