IF A MAN FINDS TEFILLIN HE SHALL BRING THEM IN. . . THIS APPLIES TO OLD ONES BUT IN THE CASE OF NEW ONES HE IS EXEMPT.
Why the difference between old and new? They may or may not be kosher. This brings us to a discussion of determining the proper status of a set of Tefillin (a set being one for the arm and one for the head):
R. Hisda citing Rab ruled: If a man buys a supply of tefillin (for trade) from a non-expert he must examine two tefillin of the hand and one of the head, or two of the head and one of the hand.
Remember that the examination of Tefillin is a tedious process. To be done correctly one has to open them and examine the text inside, then sew them back together.
If the examination finds these three to be good, the entire supply is deemed to be good. But why two of one kind and one of the other?
If he bought them from one man, why should he not examine either three of the hand or three of the head, and if he bought them from two or three persons, should not each one require examination (of each one)?
Since examination of goods from one supplier would tell nothing about those from another supplier.
The fact is that he bought them from one man, but it is necessary that his reputation shall be established in respect of those of the hand as well as those of the head.
But did not R. Kahana learn: In the case of tefillin one examines two of the hand and of the head? — This represents the view of Rabbi who laid down that if something has happened twice presumption is established.
Disagreement continues and in some cases examination of each individual set is required. But interesting that random sampling and presumption of continuity is established – even in something as important as religious items.