Reprinted with permission from Maimonides’ Introduction to His Commentary on the Mishnah, translated and annotated by Fred Rosner, and published by Jason Aronson.
[Judah the Prince] then subdivided the treatises in Seder Tohorot (purities) and began with tractate Keilim (vessels). Its subject matter includes the enumeration of all the primary ritual defilements and that which is subject to defilement, as well as that which cannot become defiled so that when we later speak of things that defile we will know which are the things that are susceptible to ritual defilement and which are not.
After Keilim comes tractate Oholot (tents), and its subject matter deals with defilements conveyed by a corpse. This tractate is first because it deals with the highest degree of defilement. Following this is tractate Nega’im (plagues, leprosy) whose contents deal with the defilement of leprosy because a leper conveys defilement through a common enclosure [tent]. Thus, it is somewhat similar to the defilement of a corpse, as will be explained in its proper place.
After completing discussions of the defilements by a corpse and the like, he began to describe the subject of purification from the aforementioned, namely the red heifer. Thus after Nega’im, he placed tractate Parah (the red heifer). Having concluded the discussion of high degrees of defilement and the manner in which one may become purified therefrom, he speaks of lesser degrees of defilement, which require only the setting of the sun [for purification].
He thus placed tractate Tohorot after Parah. It is called Tohorot (purities) using euphemistic language, because therein are described the laws of defilement. In addition, knowledge of defilements brings one to knowledge of purification therefrom. If someone would think that the reason for calling the name of the whole Order "Seder Tohorot" and the use of the same appellation for one of the tractates thereof Tohorot is incorrect we would answer no, it is not unusual for men of ideas to call a particular item by the name of the general category that includes it.
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