The first perek of Massekhet Kiddushin focuses on “acquisitions” – beginning with “acquiring” a wife, and continuing with discussion of other types of purchases. This stems from the fact that according to Jewish law, marriage is a type of kinyan, involving an act of acquisition. The Mishna teaches that a wife is “acquired” by her husband by means of three methods (the Talmud Yerushalmi makes clear that the intention is one of three methods):
In kesef the man offers the woman money or an object of value, stating that it is for the purpose of marriage. In shetar the man gives a document to the woman that indicates that he is marrying her with this paper. In bi’ah an act of sexual intercourse is performed for the express purpose of consummating marriage.
According to the Rambam (Hilkhot Ishut, 1:2) it appears that only the second two methods – shetar and bi’ah – are of biblical origin, while kesef (which is the method most often used) is mi-divrei soferim – of rabbinic origin. Virtually all of the rishonim disagree, arguing that kesef is biblical, as well. Some suggest (see, for example, the Me’iri) that the Rambam later changed his mind, while according to others, the Rambam does not really mean to say that kesef works only on a rabbinic level. They explain that the Rambam distinguishes between laws that are clearly written in the Torah (or are derived from a straightforward reading of the Torah) and those laws that are derived from some of the hermeneutic devices applied by the Sages. Halakhot derived in that manner are referred to by the Rambam as divrei soferim even though they have the same level of seriousness and severity as laws that are clearly learned from the Torah.