For example, one litigant claims that a person borrows money without a contract. The other says he borrowed it and returned it. We believe the second person, since had we wanted to lie, he would have said the loan never occurred. The exception to this is where there are eidim. Migo bimakom eidim lo amrinan - we don't say the law of migo in opposition to eidim.
Witnesses have greater credibility than that gained be migo.
Tosafos, on Baba Kama 72b, rule in the case where one side has eidim in its support, and the other has both eidim and migo. Migo is weaker than eidim, so they conclude that adding migo to his case would not help him. The last notion we will look at is that of Chazakah. For example, if a mikvah is found to be deficient of the required 40 se'ah of water, we do not say that everyone who used the mikvah since it was last checked must return to the mikvah. Rather, by chazakah dimei'ikarah, we assume that the mikvah remained in its kosher state as long as possible.