On p. 303 of Readings, Xunzi considers the following objection: “Ritual and the standards of righteousness and the accumulation of deliberate effort are people’s nature, and that is why the sage is able to produce them.” It’s probably not right to think of Mencius as the main target in this section (considering that Mencius is not named, and Xunzi was not shy in naming Mencius as the opponent in other sections of the chapter). Rather, the opponent may be thought of as someone who thinks that there is good nature, and there is bad nature.

But let me first rephrase the objection as follows:

(1) A person comes to be a virtuous agent by his (accumulated) deliberate effort.

(2) The best explanation for how (1) is possible is that virtue is already part of his nature.

(3) Virtue is part of at least some people’s nature. (From (1), (2))

Xunzi’s reply is an argument by analogy, attacking (2):

(4) Clay comes to be tiles by the potter’s deliberate effort (shaping); wood comes to be utensils by the craftsman’s deliberate effort (carving)

(5) Neither the material (clay, wood), nor the final form (tiles, utensils), of the products are part of the potter or craftsman’s nature.

(6) The best explanation of (4) can’t be that either the material (clay, wood), or the final form (tiles, utensils), of the products are part of the potter or craftsman’s nature. (From (4) and (5))

(7) The best explanation for how (1) is possible is not that virtue is already part of our nature. (From (6) by analogy, showing that (2) is false)

He then follows up with this additional argument:

(8) The virtuous (e.g., Yao and Shun) and the vicious (Jie and Zhi) agents have the same nature.

(9) We find honorable something in the virtuous agent—his virtue, and we consider base something in the vicious agent—his viciousness.

(10) Our different attitudes between virtuous and vicious agents cannot be explained by a difference in nature between them. (From (8), (9))

(11) What we find honorable in the virtuous agent—his virtue—cannot be part of his nature. (From (8), (9))

The conclusion he draws, restating his position, is that “people’s nature is bad, and that their goodness is a matter of deliberate effort”, bringing us to the bottom of p. 303. What follows reinforces the points he already touched on. He appeals to a few assumptions commonly held among fellow Confucians:

(12) Zengzi, Minzi Qian, and Xiao Yi displayed exemplary virtue (filial piety in particular), while the masses did not; the people of Qi and Lu were especially civilized, while the people of Qin were morally backward.

(13) Nature comes from Heaven.

(14) If Zengzi, Minzi Qian, and Xiao Yi (and not the masses) had exemplary virtue, and the people of Qi and Lu (and not Qin) were especially civilized, because of their nature, then Heaven favored Zengzi, Minzi Qian, and Xiao Yi over the masses, and the people of Qi and Lu over the people of Qin. (From (12), (13))

(15) Heaven does not favor one group of people over another.

(16) It’s not the case that Zengzi, Minzi Qian, and Xiao Yi (and not the masses) had exemplary virtue, and the people of Qi and Lu (and not Qin) were especially civilized, because of their nature. (From (14), (15))

Rather, Xunzi proposes, the better explanation is (given his larger account), is that the virtuous are so because they overcame nature through their accumulation of deliberate effort, while the others are the way they are because they “follow along with their inborn dispositions and nature, take comfort in utter lack of restraint, and are lax in regard to ritual and the standards of righteousness.”