The manas cannot possibly discern only the significant mental sense-objects (dharmas).

Question: Someone says: “意如刀剑锋，不能自割自者 [The mind is like the sharp edge of a sword; it cannot cut itself] refers to the seventh consciousness, the manas, also called the manas-vijñāna. The Buddha called this seventh consciousness the manas (mind root) because it is the dynamic force by which the seeds of the mind consciousness manifest from the eighth consciousness, and because the mind consciousness can manifest only by relying on the manas' manaskāra; after it has manifested, it likewise runs entirely by relying on the manaskāra of the manas. Therefore it is said that the manas is the root of the mind consciousness. The object-specific wisdom of this manas is extremely weak; it can only make an extremely simple discernment of the mental sense-objects upon the five sense objects—for example, whether there are major changes in the mental sense-objects upon the five sense objects. This seventh consciousness, unlike the mind consciousness, the knowing mind, cannot nimbly operate upon the five object-specific mental factors; it lacks the mental factors of ‘desire (chanda), determination (adhimokṣa), mindfulness (smṛti), and concentration (samādhi),’ and its function of the wisdom mental factor (object-specific wisdom) is also extremely weak; it can only make an extremely simple discernment of changes in the mental sense-objects.

Being thus, since it cannot even discern the objects of the five sense objects and must summon the mind consciousness, and only by relying on the mind consciousness's object-specific wisdom can it make various deliberations upon the various states—how could it have the ability to introspect upon itself? How could it have the ability to reflect upon the various dharmas? How could it have the ability to correct its own mental actions and habitual tendencies? Therefore, although this seventh consciousness is extremely nimble and can universally take all dharmas as object, and can, relying on the mind consciousness's object-specific wisdom, be the master everywhere and the master at all times, and can further deliberate and decide the various mental actions; yet if it is apart from the mind consciousness's object-specific wisdom, it can accomplish nothing. By virtue of such a nature, the Buddha said that the mind (the manas) is like the sharp edge of a sword (a metaphor for its nimbleness in universally taking all dharmas as object; although the mind consciousness has object-specific wisdom, it cannot universally take all dharmas as object), yet it cannot cut itself by itself (a metaphor that, lacking the ‘self-validating cognition’ of object-specific wisdom, it cannot correct its own wholesome and unwholesome mental actions). This means that for this consciousness to ‘want to change its defiled nature and transform into a pure manas,’ it must rely on the mind consciousness's object-specific wisdom and deliberative wisdom before any transformation can occur; it cannot eliminate its own corresponding afflictions by its own functions alone. Therefore the Buddha said: 意如刀剑锋，不能自割自。” 
Something about the passage above always feels off to me; could you analyze it and explain? 

Answer: Saying that the manas only discerns major mental sense-objects contradicts the Buddha's statement that the manas silently encompasses all dharmas. Since the manas can universally take all dharmas as object, then any dharma whatsoever can be taken as object; there is none it cannot reach. Yet the passage above says the manas only takes as object the mental sense-objects upon the five sense objects, and moreover only the major, changing ones. With so many limitations, with so many dharmas the manas cannot universally take as object, this contradicts the principle stated by the Buddha that the manas universally takes all dharmas as object.

For example, in contemplating the sun, one may contemplate for an hour, or for half a day or a whole day. Within an hour the sun undergoes no change, so why can the mind consciousness keep contemplating it? Does the manas not take as object the sun, which never changes? Only when the manas takes a dharma as object does the mind consciousness become qualified to discern that dharma. Whatever the mind consciousness knows, whatever the six consciousnesses discern, are all dharmas that the manas has taken as object and decided to discern. How could the manas only take as object the mental sense-objects upon the five sense objects, and only the major, changing ones at that? The Śūraṅgama Sūtra says that the manas silently encompasses all dharmas. If the manas could not take as object the subtle and minute objects of the six sense objects, then it would not be silently encompassing all dharmas, and consequently the six consciousnesses would never know those subtle and minute objects of the six sense objects.

If the manas had no desire (chanda) and did not wish to act, the six consciousnesses would not be generated at all, and none of the myriad dharmas would appear. When, amid a mass of colors, the eye consciousness selects only the color purple, that is decided by the manas. This shows that the manas also takes the objects of the five sense objects as object; only then can it act as master, letting the six consciousnesses discern that object of the five sense objects. If the manas did not take as object or contact the objects of the five sense objects, the five consciousnesses would not arise to discern the objects of the five sense objects, and there would be no capacity for choice regarding them. If there were dharmas the manas cannot take as object, then the manas would not be universally taking all dharmas as object.

If the manas could not exercise determination (adhimokṣa) and were always muddled and confused, it could not act as master, could not accomplish what is correct, reasonable, true-to-reality, and in accord with Dharma, could not avoid danger, and could not transform consciousness into wisdom. If the manas had no mindfulness (smṛti), it could not bring the mind consciousness forth to be mindful; then no dharma could appear— one could not recollect the Buddha (buddhānusmṛti), would not want to investigate Chan, and the myriad dharmas would not appear. If the manas could not deliberate, then the mind consciousness's perfuming of the manas would be meaningless, let alone transforming consciousness into wisdom, and no wisdom of any kind would ever arise. If the manas had no concentration (samādhi), then no matter how the six consciousnesses cultivated meditative concentration, they could never become concentrated.

If the manas could not correct wholesome and unwholesome mental actions, then mental actions could never be changed—the unwholesome would remain forever unwholesome. How then could afflictions be subdued and eliminated? The mind consciousness's perfuming of the manas would likewise have no effect whatsoever, and all efforts would come to nothing. The passage above says the manas has no power of introspection and no self-validating cognition, yet the Buddha said that all eight consciousnesses possess self-validating cognition. That sentient beings trust themselves so much and are so obstinate is precisely the self-validating cognition of the manas in operation. Being stubborn to the death and believing without doubt—this is the manas' self-validating cognition; it simply believes itself to be right.

The manas has the function of continual scrutiny and deliberation: all dharmas must pass through His review and approval before they are ratified and a choice can be made. If the manas' wisdom were always so lowly and weak, how could it review and examine things reasonably, how could it make wise choices? If it could not make wise choices, then sentient beings' bodily, verbal, and mental actions would everywhere and at all times display foolishness and lack of wisdom—how could there be intelligent people in the world? How could those who study Buddhism open wisdom and become Buddhas? Moreover, if the manas is to deliberate, it should deliberate by relying on its own inherent wisdom; how could it deliberate by relying on the mind consciousness's object-specific wisdom? This claim is far too strange—it is like saying that when A thinks through a problem, he must rely on B's wisdom and borrow B's brain, which does not conform to logic in the slightest. That passage above contains far too many errors; when there is time later, I will dissect it in detail.