Yevamot 90 - Can the Sages change the Torah?
On the previous page we saw a situation where the Sages declared a child of a woman a mamzer . How can this be done? Not only it is a punishment, for he now cannot marry a regular Jewish woman, but that perhaps even means that as a mamzer he can marry a woman who is a mamzer. If in truth he is kosher, and only the Sages declared him a mamzer as a penalty, then by this punishment they allowed him to transgress the law of the Torah. If that is so, the Sages would be allowed to change the law for the policy reasons. Is that possible?
Consider all the situations where the Sages take away person's property as a penalty. Are they changing the property laws of the Torah? - No, since the Torah itself gave the Court the authority to declare someone's property ownerless .
Another situation: the shofar is not blown on Shabbat of Rosh Hashanah. Did the Sages change the Torah here? - Not really, they told not to do something (literally, to sit and do nothing), and this is different from actively transgressing.
Yet another attempt: if a husband gives a Get to his wife, but then annuls it, the annulments is valid - even though this creates terrible consequences for his divorcee who may not be aware of it. Rabbi Shimon says that the annulment is not valid, due to the power of the Court. So to strengthen the power of the Court, the Sages changed the Torah law and permitted a married woman to another man? Not true either, since every marriage contract has the phrase "with the agreement of the Sages," and if they later disagree, they annul the marriage retroactively, so that the Get is not even needed. But tell me, this is where there was a marriage contract. What if one performed engagement by cohabitation - which is valid ?! - Here the Sages changed the meaning of his cohabitation from engagement to pure pleasure.
The conclusion is that the Sages cannot change the Torah.
Art: Portrait Of A Married Couple By Sir Anthony Van Dyck