Doubt (known as Samasya in Hindu philosophy) is one of the sixteen categories accepted in Nyaya Philosophy. In Hinduism, knowledge is either valid or invalid. According to Naiyayikas, knowledge based on perception (anubhava) is valid. But those based on remembrance (smriti), doubt, error and hypothetical argument is invalid. These are sources of invalid cognition, because they are not sure and definite or they do not impart the true nature of the object. Doubt in Hinduism is defined as the knowledge which is both of a positive and negative nature and is opposite to the nature of an object. Thus, the exact characteristic or salient features of an object are indefinite, so that a firm decision about the object is not possible. According to Nyaya Sutra, doubt arises because of five reasons: Some common features between one object and other similar but not alike object (samana dharma) lead to doubt whether this is that or not. For Example – On seeing a block at