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Praśāstrasena’s Āryaprajñāpāramitāhṛdayaṭīkā comments on the Heart Sūtra’s phrase “all phenomena are without arising, ceasing, purity, impurity, increase, and decrease” by using the notion of buddha nature (sangs rgyas kyi ngo bo), which exists without any change in all beings, is naturally pure, and is only obscured by adventitious stains: As for [the sūtra’s] saying “without arising, without ceasing,” the subsequent existence of what did not exist before is called “arising.” The subsequent nonexistence of what existed before is called “cessation.” Since this buddha nature—the dharmadhātu, ultimate emptiness—has no beginning point, an endpoint is not to be found. Therefore, [the sūtra] says, “without arising, without ceasing.” Even when sentient beings cycle on the five paths [of rebirth in saṃsāra], buddha nature does not become stained. Therefore, [the sūtra] speaks of “purity.” Even when awakening to unsurpassable completely perfect awakening, there is no superior purity than buddha nature. Therefore, [the sūtra] says, “without purity.” Despite manifesting in the bodies of ants and beetles, buddha nature does not become smaller. Therefore, [the sūtra] says, “without decrease.” Despite manifesting as the dharmakāya, buddha nature does not increase. Therefore, it is without becoming full. Why? Because it is beyond thought and expression and thus not within the confines of measurement. Since the dharmadhātu does not arise in two ways (through karma and afflictions), it is unarisen. Being unarisen, it is without perishing and therefore is unceasing. Since the dharmadhātu is naturally pure, it is not pure and thus is without purity. Though it is naturally pure, it is not that it becomes impure [through] adventitious afflictions. Therefore, it is pure. Since there is no decrease in the dharmadhātu through the relinquishment of the factors of afflictiveness, it is without decrease. At the time of the increase of purified phenomena, the dharmadhātu does not increase. Therefore, it is without increase.
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Karl Brunnhölzl (When the Clouds Part: The Uttaratantra and Its Meditative Tradition as a Bridge between Sutra and Tant ra (Tsadra Book 16))