The Gemora states that Rav also ruled that the child is fit. There was once a man who came to Rav and asked him: "What is the law regarding a Canaanite slave or an idolater who cohabits with a Jewess?" Rav said: "The child is fit." The man said: "If so, give me your daughter to marry (the man was from such a union)." Rav replied: "I will not give her to you." Shimi bar Chiya said to Rav: "People say, 'A camel in Medea dances in a kav (a camel can dance with its four legs in a small jar - an saying which means that people invent far-fetched stories from far away places that cannot be substantiated).' We have the kav and the camel, and this is Medea, but it is not dancing (if you ruled that it is permitted, give your daughter to him in marriage)." Rav answered: "Even if he were as great as Yehoshua son of Nun, I would not give my daughter to him." Shimi told Rav: "If he were as great as Yehoshua son of Nun, even if you will not give him your daughter, others would certainly give him theirs; however, regarding this man, if you do not give him your daughter, nobody will." Rav refused to change his mind and the man did not leave Rav. Rav gazed upon the man, and he died.
The Gemora states: Rav Masneh also ruled that the child is fit. Rav Yehudah also permitted the child. A man from such a union came to Rav Yehudah and Rav Yehudah said to him: "Go to a place where they will not recognize you, and you can marry a Jewess, or stay here and marry someone of your same type. Rava told the man the same advice.
How could they give advice to go to another city where they will not be recognized? Shouldn't this be gneivas da'as?
[This response should not be construed as a Halachic ruling on the question. Rather, it is an exploration of the topic to help our readers become more familiar with the issues involved.]
There are several possible answers to this question.
(1) Some Rishonim (Rashi Chulin 94b DH Inhu and DH Ihu) hold that Geneivas Da'as does not apply if the person being fooled arrived at his erroneous conclusion without having been told anything. However other Rishonim disagree with this.
(2) Tosfos (ibid. DH Inhu) holds that if the person being fooled should have realised that there is a possibility that the situation is not as it may seem and still does not inquire further, we may assume he is indifferent with regards to this point. In our case it is possible to say that generally before a person enters into a marriage he inquires as to the lineage of the potential spouse. If people would not do so they would have indicated that they are not particular about the lineage of the spouse. The Kehilos Ya'akov (Yevamos 44 in the new editions) quotes this explanation, but asserts that it is not valid according to all opinions. This is because some Rishonim are of the opinion that even if a person could have verified a point but did not do so he may still claim a Mekach Ta'us i.e. an invalid sale.
(3) The Kehilos Ya'akov (ibid.) answers that even though a person may not agree to consider such a match in the first instance, before a person actually gets married they are generally expected to be in a position that this question would not affect their decision. Rav did not wish his daughter to come to such a position, however it would not be considered Geneivas Da'as since we must judge Geneivas Da'as according to the position at the time of marriage and at that time the woman would presumably be willing to go ahead even if she would know all the details. (This is my own interpretation of his answer. It may not be exactly what the Kehilos Ya'akov meant to say.)
(4) The Kehilos Ya'akov further suggests that according to some opinions Geneivas Da'as is only a Rabbinical prohibition and the Rabbis did not mean it to apply in cases where it would withhold someone from his obligation to get married and have children.
(5) Perhaps we may offer a further suggestion. Even though Rav undoubtedly had a valid reason for refusing to allow this man to marry his daughter, this reason was not applicable to others who were not of Rav's stature. Others who refused did so only due to prejudice. This being the case,Rav held that this man was entitled to get married without informing people of his lineage.
[I hope this response has clarified the issue. For all practical questions on the issue, please consult your local Orthodox rabbi.]
Dov Freedman