Why can't a srotāpanna have the thought 'I have attained the fruit of srotāpanna'?

Original text of the Diamond Sutra: 须菩提。于意云何。须陀洹能作是念。我得须陀洹果不。须菩提言。不也。世尊。何以故。须陀洹名为入流。而无所入。不入色声香味触法。是名须陀洹。

The Buddha asked Subhūti: What do you think about this question? Can a srotāpanna of the first fruit have such a thought, "I have attained the fruit of srotāpanna"? Subhūti said: He cannot have such a thought, World-Honored One. Why? The srotāpanna is called "one who has entered the stream," yet there is nothing entered. He does not enter into form, sound, smell, taste, touch, or dharmas of the worldly realm; only such a one truly deserves the name of srotāpanna.

A srotāpanna of the first fruit has just eradicated the view of self (satkāya-dṛṣṭi), no longer regarding the dharmas within the five aggregates and eighteen realms as the self or as real. With no view of self and no mark of self, he also does not think there is a self who has eradicated the view of self and become a person of the first fruit. There is no person who has eradicated the view of self, no person called srotāpanna, no such fruit as the fruit of srotāpanna. The fruit of srotāpanna is a provisional designation, not a truly existing dharma, and cannot be grasped as real. If it is grasped as truly existing, then there is the view of self and the view of dharmas, and such a person is not a srotāpanna. If a person has the thought "I am a person of the first fruit," the mark of self in that person is plainly evident, and he is not a person of the first fruit.

Therefore, those who all day long proclaim "I have attained the fruit, I have seen the mind," broadcasting and showing off everywhere—one can clearly see that such people have a heavy view of self and the mark of self in full; they are without doubt ordinary worldlings (pṛthagjana). If a person constantly clings to the notion "I am a sage, I am a person of such-and-such fruit, I am a bodhisattva of the three stages of worthies, I am a bodhisattva of such-and-such bhūmi, I have cultivated this and that, in past lives I was such and such, I have done such and such for sentient beings," spreading it everywhere, fearing that people do not know, hoping that people will revere him, hoping that people will prostrate themselves in worship of him, hoping that his fame will be widespread—these behaviors clearly show that this person has a heavy view of self, the mark of self in full, and the mark of an ordinary worldling in full; he is a thoroughgoing worldling.

Since the view of self has already been eradicated, there is no mark of self, no mark of "I have eradicated the view of self." The mind has emptied the self and also emptied the fruit. In the ground of actual principle, there is no such thing as eradicating the view of self, no such person as a srotāpanna. Only when the unconditioned mind appears has one truly attained the fruit of srotāpanna, a person who has realized the unconditioned dharma. If there is a self and a mark of self, then it is not unconditioned, and there is no unconditioned fruit. Therefore, to judge whether a person has eradicated the view of self, one can also judge from his words and deeds: see whether this person's speech and actual conduct, his way of doing things, show an empty mind, whether he is low-key, whether he dislikes self-promotion. If this person likes to keep "I" on his lips and shows himself off everywhere, then the mark of self is in full, he has not eradicated the view of self, and he is not a srotāpanna.