Responsa for Sanhedrin 95:2

ואביי מ"ט לא גמר מעבודת כוכבים אמר לך מידי דאורחיה ממידי דאורחיה גמרינן לאפוקי עבודת כוכבים דלאו אורחא:

thus excluding the broken-neckedheifer, which is in itself taboo. And why does Abaye not deduce [his ruling]from idol-worship? — He answers you: Normal practices are deduced from normalpractices so excluding idol-worship which is notnormal.<span class="x" onmousemove="('comment',' 'Normal' is used in the sense of 'sanctioned by law.' I.e., it is a normal (permitted) practice to make a shroud for the dead, likewise to break the neck of a heifer under prescribed conditions. But under no circumstances can idolatry be 'normal' (i.e. — permitted). Therefore, mere designation in connection with idolatry does not impose a prohibition, because, since it is abnormal (forbidden), one may repent and never use it for the purpose. But in the case of the other two, if permitted (or even obligatory), once they are designated for that purpose they will certainly be used, unless unforeseen circumstances intervene. Therefore the mere designation suffices to give them the same status as though they had actually been used. ');">²</span>

Teshuvot Maharam