Daf Hashavua Pesachim 54b
Bein Hashmoshes , we consider night, Lechumra. (Erev Shabbos,Tisha B’av etc.)
Why don’t we say every Bein Hashemoshos has a Chazaka that it is day ?
Whenever we don’t know (safek) when a change happened, Chazaka tells us to assume,it happened at the latest possible time.
Why don’t we consider Bein Hashmoshes Vadai (sure) day, the original status ?
The ( שמעתא ג' פרק ט'-י' ) *שב שמעתתאsays חזקה העשויה להשתנות אינה חזקה a
Chazaka that will definitely change is not considred a Chazaka.
A cow that was found to be Treifoh, Chazaka makes us assume that it happened the
latest possible time. All the milk from before this period is considered kosher .We have a Chazaka
because not every animal becomes Treifoh. However Every Bein Hashmoshes will eventually
become night, therefore we have no Chazaka.
*Quotes the Maharit