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The Shiur of a Revi'it of Blood

Rav Moshe Taragin

The gemara in Sanhedrin (daf 4a) cites a machloket between the Rabanan and R. Akiva regarding the quantity (shiur) of blood required to confer tumah in other items which come into contact with the blood or are found under the same roof as the blood. All agree that the 'shiur' or minimum necessary quantity of blood required to confer tumah to another object is a revi'it. This halakha is stated by the Mishna in Ohalot chapter 2. R. Akiva and the Rabanan argue, however, as to whether this shiur can be comprised of the blood of two different bodies or not. According to R. Akiva, the shiur can be comprised of blood taken from TWO different bodies while according to the Rabanan only a revi'it of blood from ONE body can confer tumah. The gemara bases this dispute on a textual debate, part of a larger question of whether to read the Torah literally or based on the oral (phonetic) tradition of interpretation (yesh eim le-masoret vs. yesh eim la-mikrah). This article will attempt to uncover a logical concept which might form the foundation of this argument.


            We might begin our analysis by questioning which of the two aforementioned positions is the most intuitive. Our answer would probably be R. Akiva's position which allows the quantity of blood to stem from two different sources. In fact we rarely find the requirement that a halakhically dictated quantity (shiur) must come from only one source. In general as the gemara itself claims "shiurim mitztarfin" (see the mishna in Me'ila (13b)) - minimum quantities can be composed from several different items which are physically joined together. If a person must eat a ke-zayit matzo he may combine quantities from different pieces of Matzo. Why then would we side with the Rabanan who maintain that the blood must come from the same body? It would seem that our inquiry should begin with discovering a basis for the Rabanan's position which is clearly not the 'simple' approach. This exact question is posed by Tosafot in Yoma (81a s.v. "kol").


            This same line of reasoning is conducted by a different Tosafot although in a slightly different format. Tosafot in Sanhedrin (4a) begin by questioning R. Akiva's position. Tosafot note that the gemara presents his position as based upon a special pasuk. If his position is so intuitive (as stated above) why was he compelled to cite a pasuk? After all special pesukim are generally required to introduce a new conceptual model which departs from the intuitive one!!!


            Essentially, the two Tosafot pose a similar question: Our intuition demands that blood from different bodies should be combined into one shiur. Why do the Rabanan argue and why does R. Akiva feel compelled to locate a pasuk as a basis for his position?


            To begin answering these questions, a fascinating distinction drawn by the Ra'avad will be explored. In Hilkhot Tum'at Meit (4:13) the Ra'avad claims that if a revi'it of blood within a house is scattered, so that a complete quantity no longer exists in one place, no tumah is conferred to a person standing in the house. This lack of tumah conveyance occurs even though a person finds himself under the same roof as a revi'it of blood. Even though technically the entire quantity of blood is under the 'ohel' (the roof) since the blood does not exist as ONE sum, no tum'at ohel exists. This in contrast, the Ra'avad claims to other forms of tumah. If a ke-zayit of flesh from dead person is found under the same roof, even if it is divided, a person standing under the roof receives tumah. Said otherwise, a standard shiur can be combined even if there is no physical contiguity, as long as it if found under the same roof. When it comes to the shiur of a revi'it of blood though, unless the blood is physically contiguous the quantity cannot be combined. What might account for this difference?


            The Ra'avad's own explanation is illuminating. He writes: "Since blood is referred to by the Torah as nefesh (containing the life force - see Vayikra 17:11) and a revi'it is the minimum quantity needed to sustain a child, it cannot be considered nefesh unless it is physically joined." The Ra'avad bases his unique position about the requirements of the revi'it of blood upon the character of this shiur. In general, a shiur represents a minimum amount necessary to determine a significant mass. Less than a ke-zayit of an item is a trivial amount and not regarded by Halakha as sufficient to drive halakhot which revolve around eating. Less than a shvei peruta is not significant enough value to be considered meaningful in monetary halakhot. Less than a revi'it of wine is not a significant enough volume to serve for kiddush. Evidently, according to the Ra'avad the shiur of revi'it of blood is different. Blood per se does not confer tumah; rather a dead body (or a part thereof) which once contained life (nefesh) confers this tumah. As the Torah recognizes the blood as containing the 'nefesh' it too can convey tumah. However, in order for the blood to reflect the status of 'nefesh' it must be capable of sustaining life. Only a revi'it of blood is capable of sustaining this life (since this quantity can sustain life in a child). Hence less than a revi'it is not considered 'nefesh'. As opposed to merely quantitative shiurim, the revi'it shiur of blood determines its CAPACITY to be considered nefesh and to be capable of conferring tumah as a nefesh. Hence according to the Ra'avad if the blood is not physically contiguous no nefesh exists even though technically a revi'it of blood may be located under the same roof. For blood to be considered nefesh it must be one body of a revi'it.


            By contrast, a food quantity is considered food even if less than a ke-zayit - the shiur ore minimum mass is not essential to confer its inherent status. Only a ke-zayit is considered halakhically significant. Similarly in the case of a ke-zayit of flesh from a human body, its MASS does not determine its IDENTITY - rather its quantitative significance. Hence, if two separate pieces of human flesh are found underneath the same roof, even if they are physically disjointed, they combine to form the necessary shiur for conferring tumah.


            The same logic might be employed to justify Rabanan's position about one revi'it of blood taken from two different bodies. Indeed, had the shiur of revi'it been a 'standard' one we would clearly expect blood taken from different sources to combine into one halakhic mass. However, revi'it is necessary so that the blood is considered 'nefesh' and it cannot be considered such if the revi'it derives from two different humans. Stated otherwise: Two halves of a revi'it from two different people entail two halves of a nefesh which does not convey tumah. To address Tosafot's concern even R. Akiva (who ruled that the two quantities DO combine) was forced to supply a pasuk ; the combination rule when applied in the case of a revi'it of blood is by no means assumed. (See afterward for an elaboration upon R. Akiva's position).


            This notion of the difference between the shiur of a revi'it of blood and the rest of the shiurim might also explain an issue raised by the Mikdash Dovid (Rav Dovid Rappaport a twentieth century commentator who was brutally executed by the Nazis). In chapter 49 he poses the following question: The Mishna in Ohalot rules that if a revi'it of blood congeals it continues to confer tumah. Physically, however the congealed blood's mass is less than a revi'it (based upon a gemara in Shabbat daf 77). Given our previous assertion about the unique role of the shiur of revi'it the answer appears obvious (and indeed the Mikdash Dovid supplies this answer). The shiur of revi'it is not a definite quantity necessary to lend the item halakhic significance. Only a revi'it of blood is considered nefesh capable of conveying tumah. Once a revi'it of blood has been isolated we classify it as 'nefesh'. If that exact quantity of blood now congeals, even if EMPIRICALLY the mass is now less than revi'it, we are still certain that this VERY mass contains nefesh (since no blood has been lost rather the loss of size is due to a physical change). Hence the blood, as nefesh, is capable of conferring tumah.




            We have witnessed the fact that several halakhot governing the shiur of blood necessary to confer tumah depart from parallel halakhot of otehr shiurim. It is possible that this reflects the thematic difference between standard shiurim and revi'it of blood. A standard shiur lends halakhic significance to a mass of an item. The revi'it volume, however, determines the blood's definition as nefesh. Hence the shiur might have to stem from one source, be physically contiguous, but alternatively might be preserved even when physical congealing has reduced the actual mass.




1. Often the simplest way to begin analyzinig a halakha is by questioning to what degree is it a novel catgeory or is it merely a reworking of conventional ones. Is revi'it a standard shiur or does it reflect a different feature or trait of the blood.




            In analyzing the machloket between Rebbi Akiva and the Rabanan we focused primarily upon the Rabanan who differentiate between blood and other items; departing from the norm, blood cannot be combined from different sources.  hat might Rebbi Akiva's position be? If the blood can be combined does this necessarily mean that the shiur of revi'it is a standard one merely to achieve significant mass? Or might we claim that EVEN Rebbi Akiva agrees that a revi'it determines the existence of 'nefesh' BUT nefesh can be established by combining blood from different sources as well? Remember Tosafot in Sanhedrin who called our attention to Rebbi Akiva's providing a pasuk for his halakha. If the combination were based upon standard 'shiur behavior' such a pasuk would not be necessary (according to Tosafot's question). Might the citing of a pasuk indicate that even according to Rebbi Akiva the shiur is unique and we might think that nefesh only emerges from one source? The VERY PURPOSE of the pasuk is to dismiss this notion - blood taken from different sources CAN be considered a nefesh as long as the sum is a revi'it.




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