The Nyayasutras define "upamana" as the knowledge of a thing based on "its likeness to another thing which is familiar". It differs from "Anumana" (inference) in lacking a direct or immediate causal relation. It differs from "Pratyaksha" (perception), states the text, in using a linguistic referent and the foundation of pre-existing knowledge within the individual and what he learnt from his teachers, friends, family and past knowledge inherited from the wise, through a process of social cooperation. The "Upamana" method is secondary, it relies on perception, combined with linguistic referent and context. Comparison is not isolated "pramana" means, and sometimes works together with the "Anumana" and "Sabda" epistemic methods. Comparison is, in Nyayasutras, the process of permeating or infusing hypothesis, examples and tests, thus leading to objectivity and correct knowledge about something new and what one already presumes to know.