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When either a human or a k'li is immersed, there should not be any intervening substance between them and the water. If there is an any intervening substance between them and the water - for example, there was dough or mud clinging to the flesh of a person or the substance of a k'li - the person or the k'li is impure as it was originally and the immersion is invaild.

According to Scriptural Law, if there is an intervening substance covering the larger portion of the person's body or the larger portion of the k'li, the immersion is invalid, provided he objects to the intervening substance and would like to remove it. If, however, he does not object to it and does not pay attention whether it would be removed or not, it is not considered an intervening substance, even if it covers the major portion of the person or the k'li. Similarly, if it covers less than half a person's body, it is not considered as an intervening substance even if he objects to it. According to Rabbinic decree, any intervening substance that one objects to invalidates an immersion even if it covers only the lesser portion of the person's body. This is a decree, lest such a substance cover the greater portion of the person's body. And any intervening substance that covers the larger portion of the person's body disqualifies the immersion even if he does not object to it. This is a decree, lest a substance that one objects to cover the greater portion of his body.

Thus if even a drop the size of a mustard seed of an intervening substance, e.g., dough, tar, or the like, was on a person's flesh or on the substance of a k'li, and he objects to it, his immersion is disqualified. If he does not object to it, the immersion is acceptable unless the intervening substance covers the major portion of the k'li or the person, as explained.