Is the dharma that sentient beings ascertain in their minds necessarily correct?

Is the dharma confirmed in sentient beings' minds necessarily correct? No, it is not. If it were, the Buddha would not have told sentient beings: "Your mind is not to be believed; only after attaining the fruit of an arhat may you trust your own mind." Here, the fruit of arhat refers to the ultimate fruit of the Small Vehicle, the fourth fruit; the third fruit still falls a bit short. Confirmation means that the manas affirms and acknowledges something, forming an uninterrupted attention (manaskāra) that penetrates to the marrow, unshakable, and profoundly unknowable. For example, the view of self (satkāya-dṛṣṭi): sentient beings' manas confirms this, deeply rooted and unshakable—this is called self-grasping (ātma-grāha). 

Trusting one's own manas without doubt, being extremely certain of oneself—this is the manas's confirmation. Yet the confirmation of ordinary sentient beings carries great faults and immense perils of birth and death. For instance, ordinary beings all confirm that the five aggregates (pañca-skandha) are the self; such confirmation is a wrong view/false view (mithyā-dṛṣṭi), not right view. Since beginningless kalpas, ordinary sentient beings have always confirmed that their own views are correct and without error, yet throughout beginningless kalpas they have transmigrated through birth and death because of erroneous and inverted views, receiving all manner of evil retributions and bitter retributions. 

"Your mind is not to be believed"—the Buddha used this not only to admonish ordinary sentient beings, but also to admonish those of the first, second, and third fruits, because before the fourth fruit, one's cognition of the emptiness of the world of the five aggregates is still not adequate or ultimate; the view of self has not been fully eradicated from one's thoughts and concepts, and self-grasping remains. The arhat of the fourth fruit has completely eradicated the view of self, is thoroughly empty, and has eliminated self-grasping; only then is his conceptual understanding of emptiness pure, and only then may he trust his own mind. This understanding of emptiness is thorough and ultimate only within the scope of the Small Vehicle dharma; compared with the Great Vehicle dharma, it is still very incomplete and non-ultimate.

Therefore, whether an ordinary being has attained a fruit or not, generally speaking, his wisdom is insufficient for self-examination. First, his period of practice is short and his experience inadequate; second, he has seen and heard little and has nothing to compare against, so misjudgment and error are very easy. Moreover, because there is a self in his mind and he cannot separate from attachment, he will show favoritism toward and overestimate himself, easily giving rise to false enlightenment and false speech—and cause and effect never fail.