A key condition for enlightenment is that it must be possible for spiritual qualities such as compassion and wisdom to be developed until they reach a state of perfection.
The latter point centers on the proposition that the improvement of spiritual qualities is not innately subject to any limitation. The arguments that Dharmakirti presents in favor of this thesis are meant to counter the commonsense notion that, although mental qualities can be improved to a certain degree, these improvements cannot be developed beyond the boundaries of human nature. Concordant examples for such a view are the physical limit to how far an athlete can jump and the natural limit to how hot water can be heated.
The main Buddhist argument that is put forward to counter this belief is called “a reason that relates to special qualities.” There are two special qualities: (1) the mind is a “stable basis” that permits improvements to occur continuously over an indefinite period of time, and (2) such improvements “occur naturally,” making it possible for them to reach a state of perfection. Heated water is an example of an unstable basis, both because its temperature cannot rise above the boiling point and because, if the heat is maintained continuously, it will eventually evaporate. Similarly, the act of jumping may be improved through continuous training, but subsequent effort does not indefinitely continue to produce jumps of ever greater distances.
By contrast, the mind is not subject to either of these restrictions. Spiritual qualities like compassion, wisdom, etc., respond to repeated practice in such a way that each improvement of a particular quality is retained in the mind without disappearing. Therefore, whatever level of attainment one has achieved previously, the quality will continue to arise naturally and effortlessly. It is likened to the way in which, when a log of wood has been ignited, the fire will continue to burn spontaneously until the wood has burned completely. In the case of mental qualities, a direct consequence of this characteristic is that each new effort to cultivate compassion, etc., provides an opportunity for those qualities to be developed to an ever higher degree. This is what is meant by saying that the improvement of mental qualities can “occur naturally.” Tibetan commentaries explain that the mind is a “stable basis” in the sense that its essential nature of luminous awareness promotes the continuous development of compassion, etc., since these spiritual qualities arise with the same essential nature as the mind itself.
Excerpted from The Inner Science of Buddhist Practice: Vasubandhu’s Summary of the Five Heaps with Commentary by Sthiramati, translated by Artemus B. Engle