The notions of purity and impurity seem to play an important role in Buddhist axiology (i.e. theory of ethical/moral/spiritual values and aesthetical values). What is ethicallly/morally/spiritually pure is “good as means” and hence “wholesome” and a body, speech, or mind that is (perceived to be) “pure” is also considered “beautiful.” Importantly, as I often claim, according to Buddhist axiology there does not seem to anything in the world that has an “intrinsic value” but anything can be given a “extrinsic/instrumental value.”
But the purity–impurity dichotomy or duality is seen as “extreme” and hence contrarious to the Buddhist doctrine of the madhyamā pratipat (dbu ma’i lam). For example, saṃsāra is considered “impure” and nirvāṇa “pure.” So we would encounter attempts to strike a balance between such two extremes. There seem to several ways of doing that: (a) finding a mid-point, (b) finding a common ground, (c) finding their one essence or “one taste” (ro gcig), (d) cancelling the borderline between the two, (e) transcending the two poles, and so on. These might overlap with each other.
Knowing the oneness of “purity” and “impurity” theoretically is one thing but how one reacts to “purity” and “impurity” in actual life is another. Some Mantric methods require that a Mantric practitioner deliberately indulges in what is normally seen as “disgustingly impure,” so that he or she would know if the talk of “oneness of purity and impurity” has been just vain talk. So one is supposed to consume “five kind of meat” and “five kinds of nectar,” which would be viewed by any given societal or cultural standard as disgusting and outrageous. In such a conduct, the only difference between a Mantric yogin and a swine would be that the former is able to see the “oneness of purity and impurity” and the latter not.
Some Mantric sources, however, prescribe “pure substances” and some “impure substances.” So what are the criteria? Rong-zom-pa (RZ2: 599–600) gives us an insight. He states that whether a Mantric practitioner adopts or abandons “pure substance” or “impure substance” is determined by two criteria: (a) the kind of Mantric system and (b) the kind of family of Mantric deity. First, the Kriyātantric system prescribes “pure substance” but the Yogatantric system does not (necessarily). Second, practices dealing dealing family of wrathful deities (khro bo’i rigs) involve “impure substances,” whereas those dealing with the Tathāgata family (de bzhin gshegs pa’i rigs) involve “pure substances” even according to the Mahāyogatantric system.