Zevachim 58 - Sacrifice at the Top of the Altar
The correct place to slaughter the most holy offerings is in the northern part of the Courtyard. However, what would happen if a sacrifice were to be slaughtered on the actual top of the Altar? Rabbi Yose says that the sacrifice is valid as if it were slaughtered in the north of the Courtyard. Rabbi Yose ben Yehudah says that from the middle of the Altar and southward is the same as the south of the Courtyard, and from the middle and northward is the same as the north of the Courtyard.
What is the rationale for the opinion of Rabbi Yose? There is in fact a disagreement as to where exactly the Altar stood in the Courtyard, and Rabbi Yose is of the opinion that it stood completely in the north. If so, let Rabbi Yose say this directly, why did he use the words "as if it were slaughtered in the north?" Rabbi Yose wants to pre-empt your question, for you might say, that since the Torah requires " at the side of the Altar ," then perhaps on top of the Altar is not valid - and Rabbi Yose tells you that nevertheless it is valid.
Art: Caspar David Friedrich - Northern Sea in the Moonlight