We learned elsewhere: [He testified] concerning an [unclean] needle which is found in the flesh [of a sacrifice], that the knife and the hands (of the priest) are clean, while the flesh is unclean
What is this needle? One in a doubtful condition – i.e. we are not sure if it was made impure by, for example, being held by someone who had come into contact with a dead body (metal takes on the same level of impurity as the person or object it comes into contact with). Here, the sacrifice and the person performing the sacrifice are persevered from impurity by being given the benefit of the doubt – a special condition for the Temple.
But sometimes that benefit is also given:
‘We have learned [about] utensils,’ for we learned: ‘All utensils which are found in Jerusalem on the way of the descent to the ritual bath-house are unclean’, hence those [found] elsewhere are clean!
A single path led down to the mikve where unclean vessels were immersed to make them clean. Anything found on the side of the road must have been dropped on the way to the mikve and must therefore be unclean. All others must therefore be assumed clean!
Then according to your reasoning, consider the second clause: — [those found] on the way of the ascent [from the bath] are clean’, hence those [found] anywhere else are unclean?
Oh, well there is that.
Rather, the first clause is exact, whereas the second is not exact, and it is to exclude the narrow paths (where people walk the same path before and after the mikve).
But how do we know? We inquire.
Consider: this needle is an object which has no understanding to be questioned, and everything which has no understanding to be questioned . . . its doubt is clean
How then can we doubt it?
— Because it is a doubt of uncleanness which arises through a person, and R. Johanan said: A doubt of uncleanness which arises through a person, we inquire about it, even in the case of a utensil lying on the ground, just as though it were an object which has the understanding to be questioned.
So, we question the needle!