Mandukya Karika 4.14

हेतोरादिः फलं येषामादिर्हेतुः फलस्य च ।
हेतोः फलस्य चानादिः कथं तैरुपवर्ण्यते ॥ १४ ॥

hetorādiḥ phalaṃ yeṣāmādirhetuḥ phalasya ca |
hetoḥ phalasya cānādiḥ kathaṃ tairupavarṇyate || 14 ||

14. How can they, who assert that the effect is the cause of the cause and the cause is the cause of the effect maintain the beginninglessness of both the cause and the effect?

Shankara Bhashya (commentary)

The Śruti, in the passage, “When all this has, verily, become his Ātman” declares, from the standpoint of the Ultimate Reality, the absence of duality. From this standpoint of the Scriptural text, it is said: The cause,¹ i.e., the merit (Dharma) and the demerit (Adharma), etc., has, for its cause, the effect, viz., the aggregate of the body, etc. Similarly, the cause,² viz., merit and demerit, etc., is the cause of the effect, viz., the aggregate of the body, etc. How can disputants³ who maintain this view, viz., that both the cause and the effect are with⁴ beginning on account of mutual interdependence of the cause and the effect, assert that both the cause and the effect are without beginning? In other words, this position implies an inherent contradiction.⁵ The Ātman,⁶ which is eternal and immutable, can never become either the cause or the effect.

Anandagiri Tika (glossary)

¹ Cause, etc.—The birth in a body produces the effect, viz., the merit and the demerit.

² Cause, etc.—The merit and the demerit determine the birth in a body. Thus it is seen, according to this view, the cause produces the effect and the effect, in its turn, produces the cause.

³ Disputants—This is the view held by the Mimāmsakas. They maintain that the endless chain of life and death, consisting of the cause and the effect, is without beginning. It is just like the beginninglessness of the hen and the egg. This view is true from the relative standpoint.

⁴ With beginning— It is because the cause has its beginning in the effect and the effect has its beginning in the cause.

⁵ Contradiction—It is because the Mimāmsakas admitting the beginning of the cause and the effect, again assert that both are without beginning.

⁶ Ātman, etc.—The opponent may contend that the Ātman has become both the cause and the effect. The cause and the effect may have a beginning because both are the modifications of Ātman. But from the standpoint of their substratum, viz., the Ātman, they are without beginning. This contention is Baseless as the Ātman which is immutable, eternal and without parts cannot undergo any modification in the forms of cause and effect.