The Śūraṅgama Sūtra, Volume Three: The Tongue Faculty Is Originally the Wondrous True Suchness Nature of the Tathāgatagarbha

Original text: 阿难。譬如有人。以舌舐吻。熟舐令劳。其人若病。则有苦味。无病之人。微有甜触。由甜与苦。显此舌根。不动之时。淡性常在。兼舌与劳。同是菩提。瞪发劳相。因甜苦淡。二种妄尘。发知居中。吸此尘象。名知味性。此知味性。离彼甜苦。及淡二尘。毕竟无体。

Explanation: Ānanda, suppose there is a person who licks the inside of his own mouth with his tongue; after licking for a long time, he will feel fatigue. If this person is ill, his tongue will perceive a bitter taste; if he is not ill, he will perceive a slight sweet sensation. By means of the sweet and the bitter, the function of the tongue faculty is revealed. When the tongue is not moving, the mouth always has a bland taste. The tongue faculty and the fatigue—these two sense-object marks—are both signs of fatigued contact born and manifested from the bodhi mind. Because of the two delusive sense-objects of sweet-bitter and bland, knowing arises dwelling within; it draws in these sense-object appearances, and is called the nature of taste-knowing. This nature of taste-knowing, apart from the two sense-objects of sweet-bitter and bland, ultimately has no substance of its own.

Original text: 如是阿难。当知如是。尝苦淡知。非甜苦来。非因淡有。又非根出。不于空生。何以故。若甜苦来。淡则知灭。云何知淡。若从淡出。甜即知亡。复云何知。甜苦二相。若从舌生。必无甜淡。及与苦尘。斯知味根。本无自性。若于空出。虚空自味。非汝口知。又空自知。何关汝入。是故当知。舌入虚妄。本非因缘。非自然性。

Explanation: So it is, Ānanda. You should know that this knowing which tastes bitterness and knows blandness does not come from the sweet and bitter marks, does not exist because of blandness, does not issue from the faculty, and does not arise from empty space. Why is this said? If this knowing came from the sweet and bitter tastes, then when blandness appears, the knowing would vanish—how then could it know blandness? If this knowing issued from blandness, then when the sweet and bitter tastes appear, the knowing would vanish—how then could it know the sweet and bitter tastes? If this knowing arose from the tongue faculty, then in accordance with the tongue faculty's attributes there would necessarily be no sweet, bland, or bitter sense-objects; not knowing the sweet-bitter marks and the bland taste, it is therefore said that such a knowing fundamentally has no self-nature of its own. If the knowing issued from empty space, then empty space itself would possess the nature of taste-knowing, and the tongue in your mouth would not know; moreover, empty space would itself know the sense-objects of taste—what would that have to do with your tongue faculty? Therefore it is said that the tongue entrance is illusory; it is fundamentally not of the nature of dependent arising, nor of the nature of naturalness, but rather of the nature of the tathāgatagarbha.