It is believed that Seng-Ts'an, the Third Zen Patriarch and ostensible author of the Xin-Xin Ming, had leprosy, and this, he believed, was a consequence of some sin within him. Thus, when he met Huike, the Second Patriarch, he asked to be cleansed of his sins. Echoing the words of Bodhidharma which led to his own satori ("Show me your mind and I will pacify it."), Huike said, "Bring me your sins and I will cleanse them." "I have tried, but cannot find them," replied Seng-Ts'an. "Then I have cleansed you of them," concluded Huike.
He did not of course cleanse him of his sin, but brought to Seng-Ts'an's awareness that his sense of his own sinfulness was illusory, and that realization brought about his satori.
I have read commentary to the effect that, because a sense of sinfulness, a guilty conscience, is a product of self, which is itself illusory, sin is therefore likewise illusory. Sin does not exist. "If we have no self, it cannot commit sin." Although this may be in agreement with Buddhist belief, I think it might go too far. There is a vast difference between harmful deeds done (sin) and how we attach to them (sinfulness). I would suggest that we do have a self, and that it matters not that it is illusory — what is not?
Similarly, we are capable of doing harm to ourselves, the world and others, though in the context of Dao, no harm can be done. The real question is whether this 'matters' in the context of Reality, and the Daoist answer is, not at all. But because it does not matter in the context of an amoral Universe, does not mean that it does not matter in the necessarily moral sphere of human activity. We Walk Two Roads. It is illusory to deny anything in an illusory world.
To realize the meaninglessness of 'sin' in the context of Dao is to be released from the oppressiveness of a sense of 'sinfulness' and that freedom effects a change in how one behaves in the morally imbued human sphere.
Seng-Ts'an most likely could not find his sin because he didn't really have much of anything to find; what he had was leprosy. And this, he believed, was a visitation upon him of bad karma consequent to some sins committed in previous lives. And this most certainly is a most pernicious illusion. Like many of the doctrines of Buddhism, I have given up trying to understand what precisely karma is, since its definition continually morphs according to the needs of the moment (D. T. Suzuki seems embarrassed by it and suggests it was smuggled into Buddhism through its Hindu origins.), but this definition truly needs a holly-stake driven through its heart if for no other reason than that it presupposes a ‘soul’ resident in a moral universe.
As a final note, it is interesting to note that legend has Seng-Ts’an cured of his leprosy at his moment of enlightenment. Here is a wonderful opportunity to explore the nature of religiosity in contrast to spirituality.
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