רבי יוסי אומר: הסוס מטמא על ידיו, החמור — על רגליו, שמשענת הסוס על ידיו, וחמור על רגליו. ואמאי? הא קא מסייע בהדי הדדי! לאו משום דאמרינן מסייע אין בו ממש? אמר רב אשי: אף אנן נמי תנינא. רבי אליעזר אומר: רגלו אחת על הכלי ורגלו אחת על הרצפה, רגלו אחת על האבן ורגלו אחת על הרצפה — רואין כל שאילו ינטל הכלי ותינטל האבן יכול לעמוד על רגלו אחת — עבודתו כשרה, ואם לאו — עבודתו פסולה.
as we learned in a Mishnah, R' Yosei says: If a zav was riding on an animal, and there was a garment beneath it, if he was riding the horse, he renders the garment impure by means of the horse’s front legs. Only then is it impure; if it was beneath the horse’s hind legs it is not. And if he was riding the donkey, he renders the garment impure by means of the donkey’s hind legs. Only then is it impure; if it was beneath the donkey’s front legs it is not. The reason for this distinction is that a horse rests primarily on its front legs, and a donkey rests primarily on its hind legs. But why should a garment beneath the hind legs of a horse or the front legs of a donkey remain ritually pure? Don’t the legs assist each other? Is the reason not because we say that one who assists is insubstantial? Rav Ashi said: We, too, also learned support for this halakha in a different Mishnah. R' Eliezer says: With regard to a priest that stood in the Temple, and one of his feet was on a vessel, and one of his feet was on the floor, or one of his feet was on a stone, and one of his feet was on the floor, while performing priestly Temple rites; we see, if the vessel was removed or the stone was removed, whether he would still be capable of standing on one foot on the ground. If so, his service is valid. And if he could not stand on one foot, his service is invalid. Is he considered to be standing on the floor, in which case the service is valid, or is he not considered to be standing on the floor, in which case the service is invalid?