– Author: Rav Dan Cohen


In this essay, we will discuss the role of scientific studies within halachic discussions. This depends upon two issues:

  1. A) Are scientific proofs acceptable as proof of any sort within halachic discussions?
  2. B) If scientific proofs are accepted, how much weight do they carry? While the first issue has been discussed thoroughly by modern day poskim, the extent of the weight that these proofs hold has hardly been touched upon.

In this essay we will propose a halachic approach to determining the extent of the role of scientific proofs in a halachic discussion, considering that they are based on many assumptions. As a test case, we will examine the obligation to place techelet on one’s tzitzit in current times. The question of identifying the correct creature from which the techelet was produced is based upon proofs from archaeology, history, and chemistry.

The Disappearance of the Techelet

Approximately one thousand years ago, the identification of the creature from which the techelet was produced, known as the chilazon, was lost. This disappearance has had a major impact on certain mitzvot, as the Sages taught us that “only techelet that was produced from the chilazon is valid.”[3]

The disappearance of the techelet prevents the kohanim from being able to perform the avoda in the Beit HaMikdash, as their special clothes were partially made of techelet, and the absence of the chilazon from which the techelet was produced raises the question of whether the bigdei kehuna (priestly clothing) can still be manufactured.[4] As long as those clothes cannot be produced properly, the avoda in the Beit HaMikdash cannot resume.[5]

The disappearance of the techelet also prevents us from practicing the mitzva of tzitzit in its complete fashion. Indeed, the Mishna states: “The [lack of] techelet [strings] does not prevent [fulfillment of the mitzva of] the white [strings], and the white does not prevent [fulfillment of the mitzva of] the techelet.”[6] This does not mean that one is not obligated to place techelet on his tzitzit if he has the option to do so. Rather, the Mishna means that if one does not have the ability to place techelet on his tzitzit, he should still practice the mitzva with all white strings (and vice versa). This is clear from the continuation of the Mishna: “The [lack of] tefillin of the hand does not prevent [[fulfillment of the mitzva of] the tefillin of the head, and the [lack of] tefillin of the head does not prevent [fulfillment of the mitzva of] of the tefillin of the hand.” The same way that it is clearly not sufficient to only don the tefillin of the hand, so too one should not neglect placing techelet on his tzitzit if he has the option to do so.[7]

The difference between these two mitzvot is that while the avoda in the Beit HaMikdash cannot be performed due to other reasons aside from the lack of the identification of the snail that the techelet was produced from, the practice of placing techelet on the tzitzit is dependent upon this factor alone. If we could correctly identify the  chilazon, we could immediately resume the practice of placing techelet on the tzitzit and the fulfillment of this mitzva in its complete form.

The Rediscovery of Techelet

Over the years there have been a few proposals as to the true identification of the creature used to produce techelet.[8] The halachic authorities have been prompted to rule whether or not the mysterious chilazon has been rediscovered.

During the process of clarifying the identifications, many questions have been discussed: Do the proposed creatures match the indications that were mentioned by the Sages in our early sources?[9]  Did they have the ability to produce the correct color from these creatures thousands of years ago? In addition, some deeper questions have arisen: Is it possible that there is more than one snail that is valid for producing the techelet? Why do we need to produce the techelet from a living creature solely? Was the techelet lost, or was it concealed? Are we permitted to search for the techelet in the first place? We will not discuss these questions here, as our focus is on the weight of the scientific evidence alone, but those interested in fully understanding this mitzva are encouraged to further research the matter.[10]

With regard to the first question of whether the proposed creatures match the indications that were mentioned by the Sages in earlier sources, it seems clear that answering this question is not sufficient to clarify which of the proposals is the correct one (if any). The signs and characteristics given by the Sages could potentially match numerous proposed creatures (though they are also vague enough that they may not decisively match any of the candidates either), and are not sufficient to reach a clear ruling. Therefore, we must turn to scientific studies, hoping that they will provide support for one of the proposals.

Proofs from Scientific Studies

The most scientifically based proposal is that the hexaplex trunculus, also known as the murex trunculus, is the chilazon referred to by Chazal. Scientific studies provide three different types of proofs supporting this proposal: NU LIST

History – a study of ancient sources, both of our Sages and of early philosophers, point to the fact that the hexaplex trunculus is indeed the snail that was used to produce the techelet.

Archeology – Many archaeological findings suggest that snails from the Murex family, and the hexaplex trunculus among them, were used in the past for garment coloring. Pieces of clothing have been found that were dyed with the color produced from the hexaplex trunculus. Remnants of the same color have been found in jugs and clay parts. Vast piles of shells of hexaplex trunculus, the largest found in Crete and Sidon, were cracked in the exact spot that is used to extract the dye. These are only some of the findings[16]  that imply that the hexaplex trunculus is the source for techelet.

Chemistry – The indigo molecule, which is produced from the hexaplex trunculus, is the only molecule that we are aware of that is produced from a living creature and can be used to dye clothing (such as the wool of the tzitzit) in a sky-blue color. Furthermore, there are a number of plants that can be used to create indigo, which produce a color that is so similar to the color produced by the hexaplex trunculus that it cannot be distinguished by the naked eye. This includes what the Gemara calls kalla ilan (indigofera tinctoria) and the Mishna calls istis (isatis tinctoria), both known to be forgeries of the original techelet.[17]

These scientific proofs indicate that the hexaplex trunculus is the snail that was used in the previous millennium to produce the techelet.

The Weight of the Scientific Proofs

Even if all of the evidence presented in the previous section is accepted as valid, what weight do these scientific and archeological proofs carry in the overall halachic discussion, and are they enough to reach a halachic ruling that obligates us to put on this techelet?

Rav Shlomo Aviner[18] holds that scientific findings are not sufficient to determine the halacha in cases such as these. His reasoning is that scientific proofs are based upon many assumptions and uncertainties, and may in fact be proven mistaken in years to come when more scientific findings are discovered. Indeed, as of today the findings indicate that the hexaplex trunculus was used for producing the techelet, but what if other findings are discovered in future years that indicate otherwise? In his opinion, only hard facts can be taken into consideration, and not assumptions or theories.

For instance, what if the hexaplex trunculus was indeed used for dyeing garments, but was invalid for dyeing the tzitzit strings? Perhaps the piles of shells were due to the fact that people ate the snails? Even the use of the word “Murex” by Pliny the Elder and Aristotle can be explained as referring to a different snail within the Murex family, or alternatively can refer to a different species altogether.

Hence, Rav Aviner opines that the quality of the scientific evidence in this case is simply not good enough. Presumably, he holds that in order to resolve the halachic discussion, we need to have a more than 50% certainty,[19] and the scientific findings here do not reach that certainty.

On the other hand, Rav Shmuel Ariel[20] is of the opinion that scientific proofs are enough to render a definitive halachic decision. His logic is that the same way that halachic proofs and evidence are sufficient even though they often do not constitute certain or full proof, so too scientific findings carry significant weight in the halachic discussion. If we were to fear ruling based upon a scientific proof because it might be proven wrong in the future, so too we should not rule based upon halachic proofs since they might be rejected in the future by a greater Torah scholar.

In other words, according to Rav Ariel, both halachic and scientific proofs are equally valid and both are based upon assumptions (chazaka, umdena, etc.). Neither constitute complete proof, but both are enough to reach a halachic decision. In support of his opinion, Rav Ariel quotes the Ramban[21] who says: “For anyone who studies the Gemara knows that there are no full proof verifications in a disagreement between the commentators as there are in mathematics and astronomy.”[22]

An Accumulation of Scientific Proofs

Even if one accepts Rav Aviner’s opinion that scientific findings are not sufficient to determine the halachic ruling, perhaps one can argue that in a case where a number of pieces of evidence support a certain approach, they may be combined together to to render a halachic ruling.

A precedent for this matter can be found in the halacha. The Gemara[23] states that an egg about which it is uncertain whether it was laid by a terefa (a chicken with a pre-existing mortal injury or physical defect) is forbidden to eat. The Gemara continues that if this forbidden egg (due to a safek) became mixed up with even a thousand other permitted eggs, the entire mixture is forbidden.

The Rishonim[24] dispute the exact case referred to by the Gemara. According to Rabbeinu Tam, all of the eggs are forbidden only if an egg that is definitely forbidden, i.e., it was definitely laid by a tereifa, was mixed up with others. On the other hand, according to the  Ri (Rabbeinu Yitzchak), even a doubtful forbidden egg renders all of the other eggs in the mixture forbidden, since in the first case of the Gemara where it is doubtful whether it was laid by a tereifa or not, all of the eggs in the mixture are still forbidden.

The opinion of the Ri seems difficult, as the commonly accepted rule in halacha is that a safek sefeika (a double doubt) is permitted. In our case, there seems to be a safek sefeika:

  1. Maybe the egg was laid by a kosher chicken.
  2. Even if the egg was laid by a tereifa, now that it has been mixed with other eggs, perhaps we are not picking the forbidden egg at all.

If so, why is the Ri stringent in the case of a possibly forbidden egg? The Beit Yosef[25] suggests the following answer: The Ri was only referring to cases where the first doubt was known before the second doubt arose. In other words, the egg with uncertain status was initially present alone. Since there was only one uncertainty, we ruled that the egg is forbidden. Once the ruling was issued, we treat the egg as if it is certainly forbidden; therefore, even if it was mixed with others, the situation is not considered a safek sefeika. However, if both doubts arose simultaneously, i.e., the egg with uncertain status was mixed with others before it was addressed on its own, then even the Ri would agree that the egg is permitted due to the rule of safek sefeika.

We see from here that even in cases where each uncertainty on its own is not sufficient to permit the situation, if all of the doubts arise at once, they can have a cumulative effect significant enough to tilt the scale in favor of a permissive ruling. In other words, even though each uncertainty on its own was less than the 50% needed to determine the halachic decision, in certain cases we can add them together and reach the 51% (or more) majority needed to permit the item.

One could argue similarly regarding scientific proofs. Even if the premise is taken that each scientific finding does not carry enough weight to decide a halachic issue on its own, perhaps in cases where there are a number of scientific proofs, they accumulate to forge a certain probability approach (more than 50%) and render a ruling.

Applying this argument to the case of techelet in tzitzit, one can argue that there too, a cumulative effect exists. We mentioned that there are three different types of scientific proofs that indicate that the hexaplex trunculus is indeed the creature used to produce the techelet. Even if we take the position that each proof alone does not carry enough weight, adding all the proofs together may yield a very high probability regarding the identification of the chilazon. Once we reach such a high level of certainty, we can use these proofs to render a halachic ruling.[26]


In this essay we set out to determine whether scientific findings can indeed play a role within a halachic discussion. We showed that even if individual proofs do not carry enough weight because of assumptions made and doubts, we can still add the proofs together and reach a level of certainty necessary for a halachic ruling. The bottom line is that in the opinion of many poskim, scientific proofs are of no lesser quality than halachic proofs, and in both cases all we need is to reach a minimum level of certainty, not an absolute certainty.

If so, the discussion must then relate to the actual individual scientific proofs themselves. We must then evaluate whether they generate enough certainty when accumulated together or there is still a significant gap between the evidence available and the level of certainty needed.

[1] See Rav Nachum Rabinovitch’s article, “Scientific Assessment as a Basis for Halachic Ruling,” available on the Birkat Moshe Yeshiva website. See also the letter exchange between Rav Shlomo Aviner and Rav Shmuel Ariel (referred to later in this essay) on the Otniel Yeshiva website at https://otniel.org/lesson/%D7%97%D7%9C%D7%99%D7%A4%D7%AA-%D7%9E%D7%9B%D7%AA%D7%91%D7%99%D7%9D-%D7%91%D7%A2%D7%A0%D7%99%D7%99%D7%9F-%D7%94%D7%AA%D7%9B%D7%9C%D7%AA/.

[2] The exact time of the disappearance of the techelet is disputed. From the Gemara (Shabbat 26a), it is clear that the amoraim had techelet. From the Gemara elsewhere (Menachot 43a) and Tosafot (there), it is also clear that at the time of the savoraim, the techelet was still in existence. Some of the earlier sources to mention the disappearance of the techelet include the Rif (Hilchot Tzitzit 13b), who writes “nowadays that we do not have techelet,” and the Rambam in his Commentary on the Mishna (Menachot, ch. 4), who writes similarly. If so, perhaps we can assume that the disappearance of the techelet occurred at the beginning of the time of the Geonim (around 500 CE).

However, Rav Gershon Chanoch Henech Leiner, known as the Radzyner Rebbe, in his work “Sefunai Temunei Chol,” (p.9) writes that in his opinion, the techelet was still extant during the time of the Geonim. He bases this on the Rambam’s writings (Hilchot Tzitzit 2:2) that describe the snail in ways that are not mentioned in earlier sources. Another proof is that the Geonim wrote halachot regarding techelet (see Ra’avad on the Rambam, Tzitzit 1:7), and it is known that the Geonim did not address halachic issues that were not practiced at the time. One can explain the sources that hint that there was no techelet at the time of the Geonim (mentioned above) as referring to a diminution of the techelet so that “it was not practiced by the entire Jewish nation”, rather than to its total disappearance.

Another source that must be taken into account is the Midrash (Bamidbar Rabba 17:5), which seems to say that the techelet disappeared even earlier. The Radzyner Rebbe (ibid) discusses how to resolve the seemingly contradicting sources.

[3] Tosefta, Menachot 9:16

[4] From the wording of the Rambam (Hilchot Tzitzit 2:1-2, Hilchot Kelei HaMikdash 8:1) it seems that clothes of the kohanim can be made from techelet that was produced from other sources besides the chilazon. According to this, the loss of the identification of the chilazon does not prevent the manufacture of the bigdei kehuna. This is how the Radzyner Rebbe (Siftei Temunei Chol, p.14) and the Tiferet Yisrael (Kelalei Bigdai HaKodesh) understand the Rambam. However, the Tiferet Yisrael disagrees with the Rambam and holds that the clothes of the kohanim can be made only from techelet produced from the chilazon, and without the chilazon, the clothing cannot be produced.

[5] See Rambam (Hilchot Kelei HaMikdash 10:4), who writes: “A Kohen Gadol who served while missing one of these eight pieces of clothing, or a regular kohen who served while missing one of these four pieces of clothing is termed “missing clothing [mechusar begadim]” and his service is invalid, and he is punishable with death by Heaven like a foreigner who served.”

[6] Mishna, Menachot 4:1

[7] This is definitely true according to Rav Yehuda Yerucham Fishel Perlow, who is of the opinion that the mitzva of tzitzit is in fact two separate positive mitzvot – one of the white strings and one of the techelet strings. According to his opinion, one who does not place techelet in his tzitzit without good reason has neglected a positive mitzva.

According to the Rambam (Commentary on the Mishna, Menachot ch. 4) who is of the opinion that “the techelet and the white…are together one mitzva, and not two”, one who does not place techelet in his tzitzit has neglected part of a positive mitzva. This would be similar to someone who performed a brit mila for his son on the ninth day. We cannot say that this person neglected the entire mitzva of mila, but rather that he neglected part of the mitzva – the obligation of doing so on the eighth day. Likewise, one who does not place techelet in his tzitzit when he has the ability to do so has neglected part of the mitzva – the obligation to place techelet. This is clear from the wording of the Rambam (Hilchot Tzitzit 1:4): “The white does not prevent the techelet, how so? If one does not have techelet, he shall make only strings of white.”

[8] Three proposals have been suggested over the years:

  1. Sepia officinalis, which was accepted by the Radzyner Rebbe
  2. Janthina, which was accepted by Rav Yitzchak Herzog.
  3. Hexaplex trunculus, which was accepted by Rav Eliyahu Tavger.

[9] See, e.g., Gemara Menachot (34a), Sanhedrin (93a), and the Midrash (Shir HaShirim Rabba 4).

[10] Much of the information available on techelet is presented by Rav Menachem Burstein in the book “HaTechelet.” See also the sources mentioned in the footnotes in the Tzurba shiur on techelet.

[11] See Ra’avia, Berachot 1:25; Pirkei D’Rabbi Eliezer, ch. 49.

[12] Pliny the Elder, The Natural History, book IX, ch. 60, available at https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0137%3Abook%3D9%3Achapter%3D60

and ch. XXXVI, available at


line 131-


[13] Aristotle, The History of Animals, book V:

Part 15-


Chapter XIII-


[14] Some claim that we cannot rely upon the identification of Aristotle, as the English translation of “Murex” was given after the hexaplex trunculus was proposed as the origin of techelet. See https://tekhelet.com/pdf/naturaldyepaper3.pdf.

[15] It is debated whether this purpurae was a purple reddish color (the biblical argaman), techelet, or both. See HaTechelet, p.245. Both Rav Burstein (HaTechelet, p.248) and Professor Zohar Amar (“The Purple Dye Industry in Light of Archaeological Findings in the Land of Israel and Lebanon,” available at https://www.jstor.org/stable/26749351?seq=1) write that Pliny the Elder also mentioned  a bluish color that was produced from this snail. According to Professor Amar, either the different colors were produced from different snails of the same family or they were produced from the same snail using a different process.

[16] A list of many more findings can be found in HaTechelet, p.257 and onward.

[17] See Bava Metzia 61b, Sifrei, Parashat Shelach 115, among other sources.

[18] Mentioned in footnote 1.

[19] This is based on the rule used in other cases, where we determine the halacha based upon a majority of 51%. See, for example, Shulchan Aruch, Y.D. 109:1.

[20] Mentioned in footnote 1.

[21] Introduction to Milchamot Hashem.

[22] Rav Aviner’s response to the claim of Rav Ariel is that indeed both halachic and scientific proofs are not 100% certain, but only halachic proofs are accepted for reaching a halachic ruling because Hashem instructed us to use them.

Some bring proof against the opinion of Rav Aviner from the fact that some Rishonim (Semag, positive mitzvot 22; Mordechai, Menachot 969; Toldot Adam V’chavah 19:4; Agur, Hilchot Tefilin 58) bring an “archeological” proof within the discussion of whether to use tefillin of Rashi or Rabbeinu Tam. The proof was that a pair of old tefillin written in accordance with Rashi’s opinion were found in the tomb of the prophet Yechezkel. However, one can respond that this proof is not considered an “archeological” proof, as it is related directly to the mitzva of tefillin.

[23] Beitza 3b

[24] See Tosafot, Beitza 3b, s.v. “ואחרות באחרות”.

[25] Y.D. 57. The Beit Yosef is discussing a different case, where a doubtful tereifa animal was mixed with kosher animals, which is dependent upon the case of the doubtful egg being mixed with kosher ones.

[26] Indeed, the Darkei Moshe disagrees with the Beit Yosef and holds stringently that even when both uncertainties arise at the same time the mixture is forbidden, and we might claim that according to him, just as we do not combine uncertainties together, so too we do not add proofs together. However, this is not the case, and even according to the Darkei Moshe, certain proofs may be added. This is evident because the Darkei Moshe was only stringent in cases where the two uncertainties are found either in different objects or in the same object but concerning two separate issues. For instance, in the case of the egg, the first uncertainty concerns the origin of the egg and the second concerns which egg was chosen from the mixture. Clearly, there are two separate issues at play. However, when both uncertainties are present in the same object and pertain to the same issue, even the Darkei Moshe agrees that the two uncertainties combine together and permit consumption. Based on the Shach (Y.D. 110:63:4), we can argue that two uncertainties are considered to be on one issue when they are discussing one halachic principle, with both coming to answer a specific question.

According to this, in our discussion of whether hexaplex trunculus is the snail used to produce the techelet, since all the scientific evidence is brought to answer this one question, it is considered to be a “one issue” and all the proofs can be combined to offer a halachic ruling.

– Length: