Shukracharya:- A believer of guna and Karma based Varna ashram
All references are from the first chapter of Shukraniti:-
Here, Shukracharya says that individuals are responsible for shaping their own character.
72.[23] The men of the world have their luck and character according to the nature of the penance they adopt.
Here, Shukracharya said that even a person's past karma (prarabdha) is a result of their own actions.
73-74.[24] Man’s work is the cause of his good or bad luck (prosperity or adversity). Even that which is called prāktana (i.e., comes from previous birth) is really man’s own work. Who can ever be without work?
Here, Shukracharya rejects the concept of Varna Ashram based on Parents, and accepts that the division of the four varnas and the Mleccha is determined by their virtues and actions.
75-76.[25] Not by birth are the Brāhmaṇa, Kṣatriya, Vaiśya, Śūdra, and Mleccha separated, but by virtues and works.
Here, Shukracharya again refutes parents based Varna system by pointing out that everyone's ancestors are the same; then, under this system, everyone would be of the same Varna. Furthermore, he said that ancestors do not make anyone a Brahmana.
77-78.[26] Are all descended from Brahmā to be called Brāhmaṇa? Neither through colour nor through ancestors can the spirit, worthy of a Brāhmaṇa, be generated.
In the next shlokas, Shukracharya defines who qualifies as Brahmanas, Kshatriyas, Vaishyas, Shudras, and Malechchas.
79-80. The Brāhmaṇa is so called because of his virtues, e.g., he is habitually a worshipper of the gods with knowledge, practices and prayers, and he is peaceful, restrained and kind.
81-82. The man who can protect men, who is valorous, restrained and powerful, and who is the punisher of the wicked is called Kṣatriya.
83-84. Those who are experts in sales and purchases, who ever live by commerce, who are tenders of cattle and who cultivate lands are called Vaiśyas in this world.
85-86. Those men of the lower order who are servants and followers of the twice-born, who are bold, peaceful and have mastered their senses, and who are drivers of the plough, drawers of wood and grass are called Śūdras.
87-88. Those who have deserted practising their own duties, who are unkind and troublesome to others, and who arc very excitable, envious and foolish are Mlecchas.
Refuting an objection raised by the Pauranik regarding Shukra Niti.
Objection: Just before that shloka, which says that Varna is not by birth talks about praktana (karma of previous birth) hence it is birth based.
Reply: This objection is really funny. The shloka discussing praktana has zero relevance to the subsequent shloka on varna. In the praktana shloka, Shukracharya merely emphasizes that a person's past karma is the result of their own actions. Here..This concept is separate from the discussion on varna, which explicitly states that varna is not determined by birth. Plus, there is no need for unnecessary confusion when Shukracharya has clearly stated that varna is not based on birth.
Although not related to the above objection, but just for information....Since birth, we inherit a set of gunas or qualities in our life from praktana or prarabdha karma(And these intrinsic gunas have nothing to do with parents' gunas. For example, a son of a Shudra can have qualities of the Brahmana varna since birth).
And It is the intrinsic gunas derived from sanchit karma (accumulated actions from past lives), along with new gunas acquired after birth from the environment, school, etc., as well as conduct, that determine varna, not the varna of one's parents.
