When asked the essence of his method, Mencius said that it was the combining of Tao with accumulated righteousness. I am working here from Fung (The Spirit of Chinese Philosophy) whom I frankly find less than clear or insightful, and thus I am unable to share Mencius' meaning as well as I wish.
Fung defines Tao in this instance as “the principle through the existence of which men are able to attain to the highest sphere of life.” I do not know what this means. Perhaps it means Tao as a way — a method.
At first glance, 'accumulated righteousness' would seem to be just that kind of concept which would put any self-respecting Taoist's teeth on edge. But it is not about accumulating merit or good works. It is about growth.
Righteousness (yi) is a central concept in Confucian thought which means, as best as I can understand it, to do the right thing for no reason other than that it is the right thing to do. It is to always do the right thing apart from consequences, good or ill.
How we know precisely what that right thing is, is of course, problematical. But Mencius tells us that we know because we know; our heart tells us, and our heart is right. And this satisfies me, at any rate, as long as we understand this inner direction as one of principle, not of specifics.
Accumulating righteousness is becoming sage-like. It is learning, step by step, to express sagacity in our engagement with the world.
Mencius seems to have believed that he had arrived at a state of sagehood which he called the 'supreme morale' (Fung's interpretation). This was achieved through a gradual process of growth, accumulating righteousness. "It is the creation of accumulated righteousness," he wrote, "and not of righteousness snatched (Hsi — a surprise attack). He was an advocate of 'gradual enlightenment', not sudden.
Personally, I see no reason we cannot simultaneously follow both paths. The merely hypothetical promise of ‘arriving’ in either case, gradual or sudden, is mitigated by the actuality of growth.
As to whether it is possible to actually achieve some final state of sagehood, I cannot say. What I think I can say is that being a hypothetical, it is not truly germane to the actual becoming that we are. Growth in sagacity is an open-ended process of development — like life itself.
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