Aristotle: A contradiction is an opposition which of its own nature excludes a middle (Posterior Analytics, 2)
Though the contradiction excludes a middle, its contradictory statements have the middle in their ancestry.
For stating an affirmation or a negation , it is needed a rational medium, where the thoughts expressed are meant to be accepted by any participant to the discourse, both by those who would held the affirmation and those who would held its contradictory.
The previous medium is forgotten when occurs the contradiction. Since Aristotle presupposes the contradiction excludes a middle by its nature, the cause of oblivion seems to be the raise of an opposition that claims the existence of its own nature, one that is clearly apart from those sorts of nature engaged in the previous argumentation for one of those contradictory statements.
Until the clash of two contradictory statements, the argumentative medium generously exposes the nature of those who give reasons for their statements, because any argument is a rational structure that depends for its building on thinker’s effort to find the adequate premises. Nonetheless, the premises as statements that need to support the certainty of the conclusions must expose the evidential character of the things they allude to, so that they should be supported by precise insights into the nature of things.
Thus, once we come to a contradiction, we are forced to forget the previous natures known while we build premises, as they are parts of the middle excluded. However, for supporting farther the affirmation or its negation from the pair of contradictory statements, we return to such forgotten natures. There may be formulated other arguments using them, but, as a consequence of the former clash of statements, such natures are misrepresented in a conflictual way. Reasoner’s nature and the nature of the things he refers to are overestimated as realities more distinct than they really are.