True knowledge is called prama. The means to decide that a piece of knowledge is true is called pramana. Patanjali has recognized three pramanas, namely, sense experience, inference and testimony (1.7) The process of knowledge is often hampered by inadequacy of external conditions of light or by defects of the cognitive senses such as jaundiced eyes, myopia and many others.
The faculty which gains knowledge is called intellect. It always works on the basis of previous experience stored in buddhi and it makes use of three pramanas to gain true knowledge. But sometimes, due to various reasons, the intellect fails to yield true knowledge. True knowledge is called tadrupa-pratishta, i.e. in accordance with what is. What it is not so, it is called viparyaya. It is false knowledge which is not based on what is. Viparyaya can be removed and true knowledge gained by the removal of obstacles. However, one obstacle that always remains is the presence of the afflictions in the mind. Prajna is never free of them. Thus, according to Yoga, in the strict sense, even true knowledge gained by prajna from pramana (s) is faulty, as it is accompanied by avidya. When through the practice of Yoga, especially Samadhi (final stage of concentration by meditation), the afflictions are attenuated, then prajna is purified. It is called samadhi-prajna. It yields true knowledge, viveka, which shows the difference between the two basic realities. It leads to nirbija-samadhi, after overcoming all traces of ignorance and subsequently making the mind completely free of fluctuations in the state of nirodha (I.51). That state is the ideal of Yoga epistemology.
Patanjali has differentiated another kind of faulty knowledge. It is called vikalpa. It is neither true nor false. It involves a combination of words which is meaningless, in that it cannot indicate anything which actually exists in the world of experience, although the terms so combined are individually meaningful. Thus vikalpa is a meaningless, hollow combination of words. It cannot be true, because nothing exists to show its meaning. But it cannot be false either. It has an appearance of meaningfulness derived from that of its constituent terms. For example, phrases like hare’s horn (Sasha Shringa), barren woman’s son (vandhya putra) or golden mountain (hemadri). Clearly, there are no such objects, these phrases still appear to be meaningful. That feeling is derived from the meaningfulness of the constituent terms. Patanjali’s definition of vikalpa (I.9) needs to be examined carefully for arriving at true knowledge (prama).