The Śūraṅgama Sūtra, Volume Three: The Tongue Faculty as Ultimately the Wonderful 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, for example, if a person licks the inside of their own mouth with their tongue, after a long time they will feel fatigue. If this person is ill, the tongue will perceive a bitter taste; a person without illness will perceive a slight sweet sensation. Through the two marks of sweetness and bitterness, 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 marks of fatigued contact born and manifested from the bodhi mind. Because of the two kinds of false sense-dust—sweet-bitter and bland—the tongue faculty gives rise to a knowing capacity within, which draws in these sense-object images; this is called the nature of taste-knowing. This nature of taste-knowing, apart from those two sense-dusts 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: It is thus, Ānanda. You should know that this knowing capacity which tastes bitterness and knows blandness does not come from the sweet-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 capacity came from the sweet-bitter taste, then when blandness appears, the knowing capacity would vanish—how then could it know blandness? If this knowing capacity issued from blandness, then when the sweet-bitter taste appears, the knowing capacity would vanish—how then could it know the sweet-bitter taste? If this knowing capacity issued from the tongue faculty, then, in accordance with the tongue faculty's attributes, there would necessarily be no sweet-bitter or bland sense-dust; not knowing the sweet-bitter marks and the bland taste, it is therefore said that such a knowing capacity fundamentally has no self-nature of its own. If the knowing capacity emerged from empty space, then empty space itself would have the nature of taste-knowing, and the tongue in your mouth would not know it; moreover, it would be empty space itself knowing the sense-dust of taste—what would that have to do with your tongue faculty? Therefore it is said that the tongue entrance is false and illusory; it is fundamentally not of the nature of causes and conditions, nor of the nature of spontaneity, but rather of the nature of the tathāgatagarbha.