Taharos, Chapter 4, Mishnah 1. The thread of doubt and certainty in questions of tumah and taharah continues here, only now the Mishnah puts that thread into motion: it wants to know what happens when items are airborne.
The case
The opening phrase is "hazorek tumah mimakom lemakom", someone who hurls a tamei item across space, launching it from where it was to somewhere else. Two illustrations follow: "kikar levein hamafteichos", a loaf sent flying into a group of keys, and "o mafteiach levein hakikaros", a key sent flying into a group of loaves. Imagine bread lying about on the floor while a key travels through the air toward it. Sometimes it is the bread on the ground that is tamei while the key in flight is tahor, and the question is what became of the key. Sometimes the roles are reversed: the key is the tamei item and the bread is tahor, and the question is what became of the bread.
The first opinion answers in one word, covering every version of the case: "tahor". Nothing has become tamei. The justification offered is "zorek kemi she'ein bo da'as lisha'el dami", a hurled item has the standing of a party incapable of being asked anything. You cannot put questions to something in mid-flight about what it brushed against along the way; the encounter is simply beyond our ability to clarify. And wherever the doubt involves a party who cannot answer for himself, the halachah resolves the matter in favor of taharah.
Rabbi Yehudah's distinction
Rabbi Yehudah rejects a ruling that sweeps both cases together. His position: "kikar levein hamafteichos tamei, mafteiach levein hakikaros tahor". If the tumah is what sits on the floor and the item traveling toward it is tahor, the outcome is tamei. If the tumah is the traveling item and what lies on the floor is tahor, the outcome is tahor.
His reasoning limits the leniency to whatever is actually in the air. Something in flight genuinely cannot be interrogated, so when it is that flying item which carries the tumah, there is no way to establish that contact occurred, and taharah is the ruling. But reverse the picture and the flying item is the tahor one while the tumah lies motionless below. Now my worry has nothing to do with the item that traveled; it concerns those loaves resting on the ground. Tumah that never moved is outside the scope of zorek kemi she'ein bo da'as lisha'el dami, so that principle gives me no basis for ruling taharah.
The dispute therefore comes down to this. The first view treats the element of flight as enough to make the whole episode unclarifiable, so nothing in it becomes tamei. Rabbi Yehudah ties the leniency to the tumah's own position: airborne tumah yields taharah, grounded tumah does not.
An appropriate note on which to begin the chapter, and a practical caution besides: pay attention to what you are tossing and where.