Chullin, Chapter 6, Mishnah 5. This chapter is devoted to the mitzvah of kisuy hadam, the obligation to cover the blood of a slaughtered chaya (a kosher wild animal) or an of (a bird). Until now the mishnayot have dealt with who is obligated, when the obligation applies, and how it is performed. Here the mishnah turns to a practical complication: what happens when the blood that requires covering does not stand alone, but has become mixed into another liquid?
Blood mixed with water
The first case is put plainly: "Dam she'nitarev be'mayim." Blood that came under the mitzvah of covering, whether from a kosher wild animal or from a bird, has fallen into water. Everything now depends on what the eye reports. "Im yesh bo" - if the mixture before us still shows "mar'it dam," the look of blood, then "chayav lechasot," one must cover it, and the covering is done on the liquid as it stands. But if the water has taken up the redness completely, so that nothing in the vessel presents itself as blood any longer, the duty falls away, since there is no blood left to be seen.
Blood mixed with wine
The mishnah continues, "nitarev be'yayin," the blood landed in wine, and a fresh difficulty appears. Red wine is dark and red in its own right, so inspecting the mixture proves nothing: whatever redness we notice may belong entirely to the wine. The mishnah therefore hands us a test carried out in the mind: "ro'in oto" - we evaluate the case - "ke'ilu hu mayim," imagining plain water standing where the wine actually is. Leave the volumes exactly as they are, swap water for the wine in thought, and ask whether blood would be noticeable in such a mixture. Should the answer be yes, kisuy hadam is required. Should that measure of liquid have washed the color out of sight, no covering is owed.
Blood mixed with other blood
"Nitarev be'dam habehema o be'dam hachaya": now the obligated blood has run into blood that bears no obligation whatsoever, and two examples are supplied. One is blood taken from a domesticated animal, a behema, which the mitzvah of covering never reaches at all. The other is blood of a chaya whose death came about without shechitah, and that blood too falls outside the mitzvah. For each of them the ruling is "ro'in oto" - we appraise the mixture - "ke'ilu hu mayim," picturing the exempt blood as if water had taken its place, exactly the procedure used with wine. Would the obligated blood still be recognizable in that volume? If it would, we cover it; if that quantity would have absorbed it entirely, nothing remains to be covered.
Rabbi Yehuda: blood is not nullified by blood
The closing ruling draws a dissent. "Rabbi Yehuda omer," and his position is "ein dam mevatel dam": one body of blood never cancels another. In his understanding, obligated blood holds on to its identity however much exempt blood surrounds it. Let a single drop that called for covering drop into a great measure of blood that calls for none, and that drop stands there with its status intact, so kisuy hadam must be performed.
His reasoning rests on a general principle of nullification. Bitul, the halachic disappearance of a small quantity into a large one, operates when the two substances are of different kinds, min be'eino mino. When the small quantity is mixed into a large quantity of the very same substance, min bemino, there is no bitul at all. Blood mixed into blood is min bemino, and therefore, according to Rabbi Yehuda, we do not view the exempt blood as though it were water. Since min bemino eino batel, the drop that required covering is still fully present, and kisuy hadam is required.