Beitzah 18B

בשרב, ורוחץ אפילו במי משרה.

on a hot day and washes himself even in putrid water in which flax was soaked, because the heat has made him so uncomfortable.

תינח בימות החמה, בימות הגשמים מאי איכא למימר? אמר רב נחמן בר יצחק: פעמים שאדם בא מן השדה מלוכלך בטיט ובצואה, ורוחץ אפילו בימות הגשמים.

The Talmud raises another objection: This works out well in the summer season; however, in the winter, when people do not usually immerse themselves in water in order to cool off, what is there to say? Rav Naḥman bar Yitzḥak said: Sometimes a person comes home from the field soiled with mud and excrement and washes himself even in the winter.

תינח בשבת, ביום הכפורים מאי איכא למימר?

The Talmud continues with another objection: This works out well and explains why it is permitted for one to immerse himself on Shabbat, when it is permitted to wash, and therefore it can be argued that it is not clearly evident that he is immersing in order to purify himself; but on Yom Kippur, when all bathing apart from ritual immersion is prohibited, what is there to say? His actions prove that his intention is to purify himself, and this should be prohibited because it looks as though he is repairing himself through purification.

אמר רבא: מי איכא מידי דבשבת שרי וביום הכפורים אסור? אלא הואיל ובשבת שרי ⁠— ביום הכפורים נמי שרי.

Rava said: It is impossible to prohibit immersion on Yom Kippur alone, as is there anything that is permitted on Shabbat and the very same act is prohibited as labor on Yom Kippur? In fact, the prohibition against labor is more stringent on Shabbat than on Yom Kippur. Rather, since it is permitted on Shabbat, it is also permitted on Yom Kippur. The rabbis did not issue decrees with regard to Yom Kippur that would make it more stringent than Shabbat. Therefore, since they allowed a person to immerse on Shabbat because it looks as if he is doing so for pleasure, to escape the heat or to remove dirt, and not necessarily in order to purify himself, they permitted it on Yom Kippur as well, even though it is evident in that situation that his immersion is for the purpose of achieving purity.