Yadayim, Chapter 3, Mishnah 2. Our tractate deals with the tumah of hands, a rabbinic institution built on the recognition that hands touch things a person is not carefully watching. When hands contract tumah, they do not become a full rishon, a first degree of impurity; they become a sheini, a second degree. Our Mishnah now sets out which sources bring hands to that status, and then asks a sharp question: what can such hands go on to make impure?
The Mishnah's first line lays down a yardstick. "Kol haposeil es haterumah": whatever carries enough force to render terumah unfit is likewise, as the Mishnah puts it, "metamei es hayadayim", a source that conveys tumah to hands, raising them to the rank of "shniyos", a second degree. The potency of any given source with respect to the hands is thus measured by a single test: is it capable of disqualifying terumah?
Now comes the dispute. "Hayad metamei es chavertah, divrei Rebbi Yehoshua" - one hand can convey impurity to the other hand; this is the view of Rebbi Yehoshua. If a person's right hand became tamei and it then touched his left hand, the left hand is now tamei as well.
"Vachachamim omrim, ein sheini oseh sheini" - the Sages disagree. A second degree of impurity does not produce another second degree of impurity. Since the hand that became tamei is itself a sheini, it has no power to pass that status along to the other hand. This is the principle already familiar to us: impurity weakens as it travels, and a sheini is simply not strong enough to generate a new sheini.
Rebbi Yehoshua's Proof
"Amar lahem, vahalo kisvei hakodesh shniyim u'metam'in es hayadayim" - Rebbi Yehoshua pressed his colleagues with the case of kisvei hakodesh, the sacred scrolls, a Sefer Torah and the other holy writings. By rabbinic decree these scrolls carry the rank of a sheini, and by rabbinic decree whoever handles them thereby makes his hands tamei. Here, plainly before us, is a sheini producing another sheini. How, then, can the Chachamim lay down as an across-the-board principle that one sheini is never capable of generating another?
The Sages' Reply
Their answer arrived in three clauses. First: "Amru lo, ein danin divrei Torah", one does not draw a conclusion about Torah law "midivrei sofrim," from a rabbinic law. Second: "v'lo divrei sofrim midivrei Torah", and the reverse inference is equally invalid, deducing a rabbinic law from a Torah law. These are two distinct realms, and what holds true in one cannot be exported to the other. Third, and this is the sharpest point: "v'lo divrei sofrim midivrei sofrim", even one rabbinic ruling may not serve as the basis for inferring another.
That last clause is the heart of the answer. When the Sages instituted that a sheini does not produce another sheini, they were not obligated to legislate identically in every instance. The decree they enacted regarding hands and the decree they enacted regarding sacred writings are two separate rabbinic institutions, and each was given the scope and the parameters that its own purpose required. The Sages who created these rules were free to build into them whatever conditions they saw fit. Consequently the fact that sacred writings do convey impurity to the hands proves nothing at all about the general rule, since a rabbinic enactment in one area is no proof for a rabbinic enactment in another.
The lesson is worth carrying with us. Rabbinic decrees are precise instruments, tailored to the concern that prompted them, and we cannot assume that the logic of one gezeirah automatically governs the next.