Yoga Sutra - Samādhi Pada 1.8
विपर्ययः मिथ्याज्ञानमतद्रूपप्रतिष्ठम् ॥८॥
viparyayaḥ mithyā-jñānam-atadrūpa-pratiṣṭham ||8||
Error is false knowledge, without foundation.
Error is false knowledge stemming from the incorrect apprehension [of something].
Patañjali now proceeds to the second of the five different types of vṛttis, error. Vyāsa defines error as considering something to be what it is not, atad-rūpa, a state that can be subsequently removed by true knowledge of the actual nature of the thing in question. As an example he gives the perception of two moons. After consuming alcohol, a person may see double. This error of perception nonetheless produces a vṛtti in the mind of this person, but this vṛtti differs from vṛttis produced by valid sources of knowledge insofar as the seeing of two moons is an apparent perception that can be contradicted and dismissed by a later accurate perception that there is only one moon in reality, whereas valid knowledge cannot be contradicted. Vijñānabhikṣu notes that error is the result of the superimposition of wrong knowledge, mithyā-jñānam, onto an object (in our example, an extra moon is superimposed onto the actual solitary one).
The classical example of error, especially among the followers of Vedānta, is mistaking a rope for a snake: If one happens upon a rope on the path as one is walking home at dusk, and imagines it to be a snake, one is superimposing the form of a snake upon something that is not a snake. This is error according to the Yoga school (different schools of philosophy hold differing views on what constitutes error). The Nyāya school, which especially concerns itself with epistemology, the methods of accurate knowledge, has a similar definition, giving as an example of error considering mother-of-pearl as containing silver. (Specifically, Nyāya defines knowledge, pramā, as apprehending an object as it is, correctly identifying the attribute of that object, and error as the opposite, considering an object to have an attribute that in fact it does not have—the mother-of-pearl does not contain silver.) Vyāsa considers error to be essentially the five kleśas, the impediments to the practice of yoga: ignorance (avidyā), ego, attachment, aversion, and clinging to life. However, avidyā is the root of the other kleśas (II.4), and we will argue in II.5 that it is a fundamentally deeper and more subconscious type of ignorance than the surface-level error represented in this sūtra by viparyaya.