Chagigah 19A

כאן למעשר. ומנא תימרא דחולין לא בעו כוונה ⁠— דתנן: גל שנתלש ובו ארבעים סאה, ונפל על האדם ועל הכלים ⁠— טהורין. קתני אדם דומיא דכלים, מה כלים דלא מכווני ⁠— אף אדם דלא מכוין.

whereas here, the first baraita is referring to tithes, for which intention to purify oneself is required. And from where do you say this, that non-sacred food does not require the intention that one is purifying himself for the sake of eating it? As we learned in a Mishnah (Mikvaot 5:6): If a wave containing 40 se’a of water became detached from the sea and fell on a person or on vessels, they are ritually pure. The Mishnah teaches that a person is similar to vessels: Just as vessels do not intend to be purified, as they obviously harbor no intentions, so too, the case of a person is referring to a situation in which he does not intend to purify himself, thereby implying that people can be ritually purified even without intention.

וממאי? דלמא ביושב ומצפה אימתי יתלש הגל עסקינן, וכלים דומיא דאדם, מה אדם דבר כוונה ⁠— אף כלים דמכוין להו.

The Talmud rejects this proof: And from where is it clear that this is the meaning of the Mishnah? Perhaps we are dealing with one who is sitting and awaiting for when a wave will detach and fall on the vessels, and the implication is the opposite: Vessels are similar to a person: Just as a person is capable of intention to ritually purify himself, so too the case of the vessels mentioned in the Mishnah is referring to a situation where one intends on their behalf that they be purified.

וכי תימא ביושב ומצפה, מאי למימרא?

And if you would say that the Mishnah is indeed referring to one who is sitting and awaiting, what is the purpose of stating this? Such a halakha would not appear to offer a novelty; why would it be necessary to state it?