Inference proceeds from inference, and depends upon the fact that certain common qualities are found in all the members of a class, as distinguished from the members of a different class. Thus the qualities affirmed of a class will be found to exist in all the individual members of that class; this attribution of the generic characters of a class to the individual members that come under it is the essence of inference. An object perceived or inferred by a competent man is described by him in words with the intention of transferring his knowledge to another; and the mental modification, which has for its sphere the meaning of such words, is the verbal cognition of the hearer. When the speaker has neither perceived nor inferred the object, and speaks of things which cannot be believed, the authority of verbal cognition fails. But it does not fail in the original speaker, God or Īśvara, and his dictates the Śāstras with reference either to the object of perception or of inference. Viparyyaya or unreal cognition is the knowledge of the unreal as in doubt—a knowledge which possesses a form that does not tally with the real nature of the thing either as doubt or as false knowledge. Doubt may be illustrated by taking the case of a man who sees something in dim light and doubts its nature. “Is it a wooden post or a man?” In nature there is either the wooden post or the man, but there is no such fact or entity which corresponds with doubt: “Is it a wooden post or a man?” Knowledge as doubt is not cognition of a fact or entity. The illusion of seeing all things yellow through a defect of the eye (as in jaundice) can only be corrected when the objects are seen in their true colours. In doubt, however, their defective nature is at once manifest. Thus when we cannot be sure whether a certain thing is a post or a man, we know that our knowledge is not definite. So we have not to wait till the illusoriness of the previous knowledge is demonstrated by the advent of right knowledge. The evil nature of viparyyaya is exemplified in avidyā nescience, asmitā, rāga, etc.[45] Viparyyaya is distinguished from vikalpa—imagination—in this, that though the latter is also unreal knowledge its nature as such is not demonstrated by any knowledge that follows, but is on the contrary admitted on all sides by the common consent of mankind. But it is only the learned who can demonstrate by arguments the illusoriness of vikalpa or imagination.