The principle of non-contradiction is the most basic principle of human knowledge. Denying it will lead to denial of all sciences and falling into sophistry and skepticism. In the present article, we have attempted to recognize the nature of contradiction and some of its less-known aspects which may, if neglected, lead to numerous sophistries. By a precise analysis of contradiction, we conclude that the reality of contradiction is nothing but "agreement and exclusion of negation and affirmation belonging to a single affair", and this is rationally absurd both in agreement and exclusion. Although the abovementioned principle is axiomatic, reaching the dependent of contradictory statement may – in proportion to what is negated or affirmed – become complicated and, as the result, discerning contradiction will become difficult. Generally speaking, the principle of non-contradiction has three other branches each of which is applicable in a certain sphere depending on its belonging. Sometimes, what is negated or affirmed are the individual essences and sometimes what the concepts say of their instances and sometimes the truth and falsity of propositions. We have called those forms of contradiction "contradiction of essences", "contradiction of qualities" and "contradiction of propositions" respectively, and investigated the related precepts.