The Talmud uses many types of logical arguments. Some of the most common arguments and terms are discussed here.
Chazakah (presumption)
The term chazakah usually refers to the defaultassumption; i.e. what is assumed until there is evidence to the contrary. For example, if one is known to have owned real estate, it is assumed that he still owns it until proven otherwise. However, with movable items, the chazakah lies with whoever currently has the item in his possession, not with the one who had previously owned it. This principle also applies in ritual law. For example: Food known to be kosher maintains its status until there is evidence to the contrary. Also, one who engages in acts done only by Kohanim is assumed to be a kohen himself, until proven otherwise.
De'oraita and derabanan
A law is de'oraita if it was given with the written Torah. A law is derabbanan if it is ordained by the rabbinical sages. The concepts of de'oraita and derabbanan are used extensively in Jewish law. Sometimes it is unclear whether an act is de'oraita or derabbanan. For example: the Talmud says the prohibition of reciting an unnecessary berakhah violates the verse Thou shalt not take the name of the Lord thy God in vain. Maimonides sees the Talmud as proving a de'oraita prohibition, while Tosafot considers the law to be only derabbanan, and sees the Talmud's scriptural reference as only an asmachta.
Examples
Examples of the application of these two terms abound. Examples include:
Birkat Hamazon contains four blessings. While the first three are considered de'oraita, the fourth blessing was added much later on in Jewish history and is derabbanan.
Regarding the verse "Thou shall not boil a kid in its mother's milk": From this, many laws of kashrut are derived by the rabbis. One might think this would make it derabbanan because it was derived by the rabbis, but the laws are actually de-'oraita because they are derived by interpreting the Torah. However, the extension of this prohibition to eating chicken with milk is derabbanan, as it is the product of a specific rabbinic enactment.
A kal vachomerderives one law from another through the following logic: If a case that is generally strict has a particular leniency, a case that is generally lenient will certainly have that leniency. The argument can also work inreverse, and also in areas where lenient or strict might not be precisely applicable.
Migo
A migo is an argument for a defendant that he ought to be believed regarding a certain claim, because he could have made a different claim which would definitely have been believed. For example, if someone admits to having borrowed money and claims to have paid it back, he is believed because he could have claimed that he never borrowed the money in the first place.
