/Mahâyânism/ generally distinguishes two or three forms of knowledge. This classification is a sort of epistemology, inasmuch as it proposes to ascertain the extent and nature of human knowledge, from a religious point of view. Its object is to see what kind of human knowledge is most reliable and valuable for the annihilation of ignorance and the attainment of enlightenment. The Mahâyâna school which has given most attention to this division of Buddhist philosophy is the Yogâcâra of Asanga and Vasubandhu. The _Lankâvatarâ_ and the _Sandhinirmocana_ and some other Sûtras, on which the school claims to have its doctrinal foundation, teach three forms of knowledge. The sûtra literature, however, as a rule does not enter into any detailed exposition of the subject; it merely classifies knowledge and points out what form of knowledge is most desirable by the Buddhists. To obtain a fuller and more discursive elucidation, we must come to the Abhidharma Pitaka of that school. Of the text books most generally studied of the {88} Yogâcâra, we may mention Vasubandhu’s _Vijñânamâtra_ with its commentaries and Asanga’s _Comprehensive Treatise on Mahâyânism_. The following statements are abstracted mainly from these documents. The three forms of knowledge as classified by the Yogâcâra are: (1) Illusion (_parikalpita_), (2) Relative Knowledge (_paratantra_), and (3) Absolute Knowledge (_pariniṣpanna_). _Illusion._ Illusion (_parikalpita_), to use Kantian phraseology, is a sense-perception not co-ordinated by the categories of the understanding; that is to say, it is a purely subjective elaboration, not verified by objective reality and critical judgment. So long as we make no practical application of it, it will harbor no danger; there is no evil in it, at least religiously. Perceptual illusion is a psychical fact, and as such it is justified. A straight rod in water appears crooked on account of the refraction of light; a sensation is often felt in the limb after it has been amputated, for the nervous system has not yet adjusted itself to the new condition. They are all illusions, however. They are doubtless the correct interpretation of the sense-impressions in question, but they are not confirmed by other sense-impressions whose coördination is necessary to establish an objective reality. The moral involved in this is: all sound inferences and correct behavior must be based on critical knowledge and not on illusory premises. {89}