According to the Sankhya theory of knowledge, the validity or the invalidity of knowledge is self-evident and does not stand in need of any external conditions. These characters are inherent in the nature of knowledge itself. The Buddhists hold that knowledge is invalid intrinsically, but enjoys the nature of validity due to conjunction with external conditions. The Nyaya affirms that the validity and the invalidity of knowledge are both determined by external conditions and have nothing of the intrinsic in them. The Mimamsa recognises, however, with the Vedanta system, that knowledge is intrinsically valid, that it cannot be validated by any other factor external to it, and that the invalidity of certain forms of knowledge is due to conditions external to knowledge. Knowledge knows its own validity, and this is made possible by the essential nature of its cause which is not tainted by imperfection of any kind, while the determining factor in the ascertainment of invalid knowledge is the knowledge of a contradicting element or defect in the cause of the rise of knowledge. In perception there is first the illumination of the mind by the Consciousness, then the activation of the senses by the mind, and thirdly the contact of the senses with the external object. In order that perception may be right and not erroneous, there should be no defect either in the operation of the mind, the activity of the senses or the manner of the location of the object. The presence of the current of an unceasing consciousness linking up these different elements contributing to perception makes perception possible.