चित्तान्तरदृश्ये बुद्धिबुद्धेरतिप्रसङ्गः स्मृतिसङ्करश्च ॥

cittāntaradṛśye buddhibuddheratiprasaṅgaḥ smṛtisaṅkaraśca

Another cognising mind being assumed, there will be no end to such assumptions, and confusion of memory will be the result.

In plain words: If we said a second mind watches the first mind, we would need a third to watch the second, and so on forever. Memory would make no sense then. So the watcher must be the Self, not another mind.