Our Gemara notes a textual anomaly in the verse that discusses sodomy (Vayikra 18:22): The Hebrew term used is “Mishkave Isha,” which suggests plural, as in multiple forms of laying together. Therefore, the Gemara uses this plurality to deduce additional prohibited acts of sexuality even without complete entry. Although the pashut peshat might simply be that occasionally words are stated in the plural, like the royal “we,” as if to say “the ways that men lay with women,” other commentaries read different implications from this plural phrase.
A particularly interesting interpretation is that of Rabbenu Chananel, who seems to have presaged sex reassignment surgery. Here is a translation of his exact words: “There are those who will create within their body in the form of a woman’s flesh.” The implication is that even in Rabbenu Chananel’s time, the technology existed, and some individuals were able to somehow create an artificial vagina, perhaps by gradually stretching skin. This is not as far-fetched as it may seem, because we know such procedures existed to create an artificial foreskin by stretching skin, as seen in Sanhedrin 44a.
There may be a rare few individuals who are hormonally androgynous and therefore feel as if they are the wrong sex trapped in their bodies. But such conditions are relatively rare and may be caused by factors such as the mother’s or fetus’s body having an immune response, suppressing certain hormones.
Medical conditions exist where persons are born with ambiguous genitalia or genitalia of both sexes. Rabbi Alfred Cohen (Cohen, 1991) provides the following medical background to better understand the halakhic ramifications:
“In order to understand the dimensions of the situation, a little medical knowledge is necessary: Both male and female genitalia come from the same original tissue in the developing fetus, and at one time, they are identical. The only thing that causes these tissues to become male or female is the result of hormones and their actions upon the developing tissue... A hormone is defined as a substance that is made in one part of the body and influences another part. This requires the substance to be made correctly, secreted into the bloodstream or surrounding fluid properly, and that the tissue it is supposed to reach can recognize it and respond to it. This process is not dependent on the presence or absence of a Y chromosome. (The chromosome responsible for determining male sex)... On the forty-ninth day, a substance called sexual determining factor is made in embryos with a Y chromosome (the gene that codes for this substance is found only on the Y chromosome)... Sex is determined by having a Y chromosome... However, it is possible to have a case where this substance is not made in a genetic male. The result would be a female in every sense of the word, except perhaps fertility. (This is an EXTREMELY rare event). In a normal fetus, the testicles are the source of testosterone. However, there are situations where testosterone is made by another part of the body. This will transform a fetus with ovaries and a uterus to develop some outward signs of masculinization of the genitals... Depending on the amount of testosterone and how soon after fertilization it is present will determine how closely these genitals resemble male genitals... The reverse problem also exists. If the fetus cannot detect testosterone, then female genitals will develop despite the presence of testicles... It is also possible to see degrees of this problem... Scientifically speaking, a true hermaphrodite is determined by looking at tissue that is either an ovary or a testicle and finding that it is both. From a medical point of view, when a child is born with ambiguous genitalia, it is always possible to determine the genetic sex... Likewise, it is almost always possible to determine what the problem is (i.e., too much testosterone made, or if it is not being detected properly by its target tissues).”
These descriptions apply to physical conditions with physical manifestations. Most people identifying as transgender today have neither physical nor genetic indicators of any divergence from their physical sex, although some suggest that different levels of exposure to male or female hormones in utero subtly influence the brain. However, the reasonable position is that Gender Dysphoria is a psychological condition, and being transgender, which may or may not accompany Gender Dysphoria, is a form of self-identification. Whether such conditions are pathological or merely a lifestyle choice is more a matter of social convention and construct.
There is also a fascinating idea expressed in Kaf Hachayyim, Hilchos Tefilin O.C. (38:3:9). He discusses the halakhic permissibility of women wearing tefilin, based on the Talmudic tradition that Michal, daughter of King Saul and wife of King David, wore tefilin. Kaf Hachayyim raises the question as to why her wearing tefilin, a man’s vestment, was not a violation of lo yilbash (cross-dressing, see Devarim 22:5)? Kaf Hachayyim provides a novel answer (based on Kabbalistic sources): Michal knew, through divine inspiration, that she had the soul of a man.
This is a remarkable and unique answer that requires further study and raises logical questions. Even if we were to fully accept Kaf Hachayyim’s assertion as correct, the implication is that one who has the body of a female but the soul of a man would not be violating lo yilbash! How is this possible? The Torah prohibition refers to externally visible gender, not gender of the soul! How could it be sensible that a halacha be based on a person’s soul identity? The general rule is, “The hidden is for God, and the revealed for us” (Deuteronomy 29:28), and this dicta is rabbinically applied almost universally to mean that halachic rulings are based on observable phenomena (see Niddah 20b and Sanhedrin 6b). We might propose that a prohibition, when transgressed in the spirit of the correct gender, is not considered a violation. In other words, since the person subjectively believes themselves to be of a particular gender, it is acceptable. This second approach, while novel, avoids the issue of a ruling based on non-observable phenomena. How so? When it comes to matters pertaining to self-intention, there is halachic precedent for basing a decision on internal subjectivity, such as with oaths (Maimonides, Laws of Oaths 2:7, Talmud Nedarim 19a). This is not the same as basing a halachic decision on non-observable phenomena, because it concerns intent, not reality. Thus, through pilpulistic argument, we can justify Kaf Hachayyim’s logic: Halachically, in terms of various external laws dependent on others’ perceptions, Michal’s gender was female. For example, she would not be considered a male inheritor and could not serve as a witness in the capacity of a male. However, insofar as the restriction of lo yilbash, since “she” believed she had the soul of a man and therefore subjectively did not cross-dress, it was permitted. This argument is admittedly strained, but it offers a way to understand the lomdus of such a great sage. Thus, at least with regard to Michal wearing tefilin, Kaf Hachayyim allowed “her” self-definition to determine “her” gender.
One might object, arguing that having a ruach hakodesh perception of one’s soul gender is not the same as self-perception, and Kaf Hachayyim would not allow a modern transgender person to cross-dress, because such a person’s self-perception is not based on divine insight.
In any case, we do see some room, from Kaf Hachayyim, for a person to believe they have been born into the wrong body, so to speak. However, if this is true for a small and rare part of the population, it has little to do with the trans hysteria of our times, nor can it justify the sexual mutilation of minors.
While we are getting a little political, I believe Sefaria’s translation of Rabbenu Chananel is incorrect and politically motivated. The current translation is: “Some people attempt to deceive others into believing that they are women instead of men.” This completely omits the key point. Rabbenu Chananel was saying that the plurality in the verse includes prohibiting a man who constructs a vagina. The correct translation ought to be: “There are those who will create within their body in the form of a woman’s flesh.” Also see Ibn Ezra (ibid) who clearly understands Rabbennu Chananel as referring to a form of surgery. (I notified Sefaria and they said they corrected the translation.)
Returning to Rabbenu Chananel, we see that he holds the Torah forbids not only homosexuality but also a man attempting to alter his body to that of a woman.
We might ask ourselves whether this prohibition applies to any sex reassignment surgery, or is it limited to one physical change local to the genital area? In other words, since halacha follows appearance, what if the person truly and thoroughly appears female? There is a startling Yerushalmi (Niddah 10a:1) that offers a possible precedent:
“Rabbi Yesa stated in the name of Rabbi Yochanan: If the entire body is that of a human, however the face is that of a cattle-beast, it is not considered a human child. If the entire body is that of a cattle-beast, however the face is that of a human, it is considered a human child. [Therefore], in the case of a being whose entire body is human, but whose face is that of a cattle-beast, even while standing and reading from the Torah, we may say, ‘The time has come to slaughter you for food.’ Likewise, if the being’s body is that of a cattle-beast, but its face is that of a human, even if pulling a plow in the field as if an animal, we may say, ‘It is time for you to perform Halitzah or consummate the Levirate marriage.’”
Incredibly, this position in the Talmud Yerushalmi seems to assert that the very definition of whether one is considered human or beast is determined by external facial characteristics, to the point where the “person” with the face of a cattle-beast, no matter how sentient, can be slaughtered and eaten as any bull on the farm! It follows that, if so, kal v’chomer surely external, and even possibly mere facial characteristics, can be used to determine gender!
However, aside from the possibility that halacha may not align with this obscure Yerushalmi, two unresolved questions remain: 1. For a transgender person who has had sex reassignment surgery or hormone therapy, would only the “top” be required to achieve the status of a particular gender, as per the Yerushalmi, or is the “bottom” more relevant, or perhaps both? Note that Tanach and Mishna use the word “face” or face-like metaphors to euphemistically refer to lower sexual regions (Proverbs 30:20, Mishna Sanhedrin 8:1).
A second unresolved question is whether the Yerushalmi only holds this way for individuals born as such, but if a person changes surgically or otherwise after birth, we follow the original biological makeup.
One 20th-century halachic authority, Rav Eliezer Waldenberg, addressed this in his Tzitz Eliezer responsa (11:78, 25:26:6). The first responsum discusses organ transplants, raising the hypothetical case of “significant/organic alterations of the body, such as a person changing from male to female or vice versa” through rare surgery. Rav Waldenberg ruled that a woman who had masculine characteristics, apparent to the visual sense, would still be considered a man, though this decision was post-facto. In the second case, he ruled that a baby born with female-appearing genitalia but XY chromosomes should be treated as a girl by Jewish law.
Rav Waldenberg’s position was only post-facto; it did not address the permissibility of the surgery, which might fall under the prohibition of serus (Vayikra 22:24, Devarim 23:22, and Talmud Shabbos 110b). Note that this prohibition might only apply to men, not women (Rashba Shabbos 11a). Even if we accept Rav Waldenberg’s position, we lack evidence to suggest that someone identifying as trans and suffering from Gender Dysphoria would be permitted to undergo such surgery as a “cure.” Even if we argue that the distress is life-threatening, pikuach nefesh, we cannot rely on the authorities today, who are often more committed to wokeness than rigorous scientific analysis. There is insufficient data to support whether suicidality in those suffering from Gender Dysphoria would be alleviated long-term by surgery, or whether the suicidality is part of a coexisting depression that will remain despite the surgery. The current political climate has not allowed for enough objective study. As we saw, we cannot even get a correct translation of an ancient Hebrew text because of political reasons, let alone objective scientific analysis.
One final point: Even if expert physicians and scientists could provide adequate testimony that such surgeries may be life-saving, there are matters in the Torah that override concerns of life and death, such as sexual immorality. Rashi codifies the prohibition of lo yilbash as based on a violation of sexual norms, which can lead to adultery. He states (Devarim 22:4): “The apparel of a man shall not be on a woman, so that she does not look like a man in order to consort with men, for this can only be for the purpose of adultery.”
Gemara (Sanhedrin 75a) rules that when a prohibition is related to sexual immorality, even if it involves only a mild sin, there is still an obligation to martyr oneself. The case involved a love-sick man (Sanhedrin 75a), about whom the doctors ruled that he would die if his wish was not granted. Despite this, the rabbis did not allow him a dispensation. Notably, even when they suggested that it might ease his sickness to converse with her behind a fence, they still did not permit it. The fact that the prohibition was mild and not actual adultery did not influence their ruling. They either considered it to have the status of an adulterous sin for which martyrdom is required, and/or they saw it as a matter that would lead to the general denigration and disrespect of modesty customs.
If we follow Rashi’s assessment of the prohibition, even though it is only a mere lo-saa’seh, negative commandment, it may still fall under the martyrdom requirements of lo-saa’sehs of forbidden sexual behavior that is more severe.
In summation, we see that halacha provides a number of fascinating halachic precedents regarding the phenomenon of transgender changes and Gender Dysphoria, and further study is needed in terms of practical Halacha and societal implications. A final note: While, politically and socially, trans activism can be harmful and destructive, and we must allow halacha to guide and inform us over wokeism, none of these discussions should overshadow the need for compassion, respect, and dignity toward the individual who may be suffering greatly.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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Rabbi Simcha Feuerman, Rabbi Simcha Feuerman, LCSW-R, DHL is a psychotherapist who works with high conflict couples and families as well male sexual health. He can be reached via email at simchafeuerman@gmail.com