OU TORAH Yitein and Yutan(SHEMINI) By Rav Moshe Twersky, HY"D

This pasuk teaches us the halacha of hechsher mashkin, that the only way a food item can become tamei (by coming into contact with a sheretz, neveilah, etc.) is if it first came into contact with water (or one of the other seven liquids). The Gemara in Bava Metzia (22a-b) brings the following Braisah: Dew (which is one of the seven liquids) fell on some produce without the owner’s awareness. When he subsequently found out about it, he was happy. If the dew is still on the produce, the produce is muchshar (= now susceptible to becoming tamei), but if it has already dried, it is not. This Braisah, explains the Gemara, seems to disprove Rava’s opinion regarding yiush sheh’lo mi’daas being effective. If someone lost something in a way that he will for sure give up hope on ever getting it back, the finder can keep the lost object even though the original owner is still unaware of the fact that he lost it. His potential yiush, Rava maintains, is effective even before it occupies his conscious thoughts.