Kiddushin, Chapter 3, Mishnah 5. This mishnah opens with a discussion of a man who betrothed a woman and afterward claims that he performed the betrothal based on a certain assumption that turned out to be mistaken. We will see that as long as the condition was not stated explicitly, it makes no difference what the man betrothing had in mind. We will also discuss betrothal that is meant to take effect in the future, once a certain fact comes about.
"HaMekadesh es ha'ishah ve'amar kesavur hayisi":
The mishnah opens: "HaMekadesh es ha'ishah ve'amar kesavur hayisi" - a man betrothed a woman, and after the fact he claims that he had been operating under a certain assumption:
"Shehi kohenes vaharei hi leviyah" - he thought she was the daughter of a kohen, and in fact she is the daughter of a Levi.
"Leviyah vaharei hi kohenes" - he thought she was the daughter of a Levi, and in fact she is the daughter of a kohen.
"Aniyah vaharei hi ashirah" - he thought she was poor, and in fact she is wealthy.
"Ashirah vaharei hi aniyah" - he thought she was wealthy, and in fact she is poor.
In all of these cases the mishnah rules: "Harei zo mekudeshes, mipnei shelo hit'aso". Even though his assumptions turned out to be mistaken, the betrothal is valid, since the woman did not deceive him or mislead him. He never verbalized the condition, and she said nothing about the matter. This is nothing more than a thought in his heart, and that does not affect the validity of the betrothal.
Betrothal taking effect in the future:
The mishnah continues: "Ha'omer la'ishah harei at mekudeshes li" - meaning that the betrothal should take effect at a later time, when the present impediment falls away:
"Le'achar she'esgayer" or "le'achar shetisgayri" - where one of them is not Jewish at the time.
"Le'achar she'eshtachrer" - the man betrothing is currently a Canaanite slave, and he cannot marry a Jewish woman until he is freed and becomes a full Jew.
"Le'achar shetishtachreri" - the woman is currently a Canaanite maidservant, and he cannot marry her.
"Le'achar sheyamus ba'aleich" - the woman is currently married to another man.
"Le'achar shetamus achoseich" - the man betrothing is married to her sister, and therefore she is forbidden to him as long as her sister is alive.
"Le'achar sheyachlotz lach yevameich" - the woman is a shomeres yavam, who requires chalitzah from her deceased husband's brother, and until she receives it she is forbidden.
In all of these the mishnah rules: "Einah mekudeshes". Each one of these situations prevents the betrothal from taking effect, and since the betrothal cannot take effect now, it cannot take effect in the future either, for it does not take hold at all. This ruling follows the view of Rabbi Akiva, who holds that betrothal does not take hold with those forbidden by a negative commandment, that is, people who are forbidden to one another. That is why "le'achar sheyachlotz lach yevameich" is included in the list as well.
However, there are opinions which hold that even one who does not rule like Rabbi Akiva regarding those forbidden by a negative commandment - that is, cases involving only a prohibition of a lav and not kareis, such as a shomeres yavam - a shomeres yavam herself is an exception, because of the way the prohibition is worded in the verse: "Lo sihyeh eishes hames hachutzah le'ish zar" ("The wife of the deceased shall not marry outside the family to a stranger"). "Lo sihyeh" - an expression of "being," and wherever an expression of "being" is used, it is a term of betrothal. Therefore betrothal does not take hold with her in any case, independent of Rabbi Akiva's view that betrothal does not take hold with those forbidden by a negative commandment.
Betrothal of an unborn child:
The mishnah continues: "Vechein amar lachaveiro im yaldah ishtecha nekeivah harei hi mekudeshes li" - the father accepts the betrothal on behalf of his daughter, which is within his authority to do. The ruling is "einah mekudeshes", since that girl is not yet in the world.
By contrast: "Im haysah eishes chaveiro me'uberes vehukar ubarah - devarav kayamin". When the woman is already pregnant and the pregnancy is recognizable, it does take effect, "ve'im yaldah nekeivah mekudeshes" - even though at the time of the betrothal she had not yet been born.
The Gemara, however, establishes this ruling of the mishnah specifically according to the view that a person can transfer something that has not yet come into the world, meaning that a person has the power to convey and to acquire something that does not yet exist - and that itself is a matter of dispute.