Why can't an arhat have the thought "I have attained the fruit of arhatship"?

Original text: 须菩提。于意如何。阿罗汉能作是念。我得阿罗汉道不。不也。世尊。何以故。实无有法。名阿罗汉。世尊。若阿罗汉作是念。我得阿罗汉道。即为著我人众生寿者。

The Buddha said to Subhūti: What do you think? Can an arhat have the thought, "I have attained the arhat fruit"? Subhūti replied: An arhat cannot have such a thought, World-Honored One. Why is this said? Because in truth there is no dharma that can be named "arhat." World-Honored One, if an arhat were to think in his mind: "I have realized the arhat fruit," then he would be attached to the mark of self, the mark of person, the mark of sentient beings, and the mark of life span. He would no longer be an arhat.

If anyone has the thought: "I have accomplished this or that," believing there truly is a self that has accomplished this or that, then he is attached to the mark of self, and the other three marks will immediately appear as well. The same is true of an arhat: if he believes he has realized the arhat fruit, then the mark of self arises in his mind, and the marks of person, sentient beings, and life span will also be clearly present; such a person is not an arhat. In this world there truly is no dharma called "arhat." The arhat's five aggregates, composed of his physical body and conscious mind, are subject to arising, ceasing, change, and illusion; they are not an actually existing dharma and cannot be called "arhat." The arhat fruit is merely a state and a realm; there is no substantial fruit to be obtained or seen. The realization of the arhat fruit is itself an illusory dharma of arising, ceasing, change, and transformation, not an actually existing dharma. If it were an actually existing dharma, it would exist eternally, and one would eternally realize the arhat fruit without any cultivation.

Therefore an arhat has no such thought as "I have realized the arhat fruit." After realizing the arhat fruit, his mind is empty and tranquil, without even a trace of the four marks. Walking in the human world, he is like an empty shell, without any conflicts or disputes. Ordinary beings are just the opposite: due to the mischief of self-view (ātma-dṛṣṭi), the four marks abound, and they constantly dwell in afflictions and disputes. The heavier a person's self-view, the heavier his mark of self, the less pure his mind, the heavier his afflictions, and the more conflicts and disputes he has; conversely, the purer he is and the fewer disputes he has.

From the first fruit to the fourth fruit, the noble ones of the Small Vehicle all have no four marks. The Mahāyāna bodhisattvas should all the more not be attached to the four marks; likewise they would not have such thoughts as "I have seen my true mind," "I have seen my nature," "I have become a bodhisattva of the ten abodes, the ten practices, or the ten dedications," nor would they go around proclaiming "I am a first-bhūmi bodhisattva," "I am a second-bhūmi bodhisattva," "I am a third-, fourth-, or even eighth-bhūmi bodhisattva," and even less would they proclaim everywhere, "I am so-and-so a sage come again," "I am so-and-so a Buddha come again." The bodhisattva's mind is purer than that of the śrāvakas, for after all they have realized the emptiness of both the Great and Small Vehicles. The śrāvakas have only realized the emptiness of worldly marks; they have not realized the self-natured pure mind, and do not know why the worldly marks and dharmas of emptiness are empty, so their realization of emptiness is not yet pure enough. Bodhisattvas would not have these impure, unreasonable thoughts, nor would they engage in grand propaganda to win over the crowd. Those with true realization have minds that are empty and pure, attached to no dharma marks whatsoever; they are truly worthy of our respect and emulation.