Shalom,

I will try to summarize the discussion in Klal 6 of the Asvan d'Oraisa:

Rav Yosef Engel is discussing the nature of the prohibition of Demai -- the restriction against eating produce purchased from an Am ha'Aretz due to the concern that the required Terumos u'Ma'asros may not have been separated. The central question he raises is whether the prohibition of Demai is due to an inherent issue with the produce itself, meaning that the produce is intrinsically forbidden until it is definitively known that it was tithed, or whether the prohibition exists only due to the lack of knowledge on the part of the individual. If it is the latter, then the obligation to separate Terumos u'Ma'asros is merely a precautionary measure, and if it is later clarified that the produce had in fact already been tithed, then retroactively there was never an actual prohibition.

To illustrate the distinction, he draws a comparison to a case where a person believes he is eating Besar Chazir (non-kosher meat), but it later turns out that he was actually eating kosher meat. If we say that the prohibition of Demai stems from the individual's lack of knowledge, then this case is analogous: the uncertainty itself caused the prohibition, but once it is clarified that everything was in fact permissible, the prohibition retroactively disappears. However, if we say that the prohibition is an intrinsic issue with the produce, then the food remains forbidden until it is definitively known that it was tithed, even if it was actually tithed in reality. This is because the nature of the prohibition is absolute and does not depend on the individual's knowledge.

A practical consequence of this distinction is the question of whether someone who later discovers that the produce was indeed tithed is considered to have transgressed by eating it before he knew that it was permitted. If we say the prohibition stems from the produce itself, then he has consumed something that was forbidden at the time of eating. But if the prohibition exists only due to lack of knowledge, then once it is clarified that the Terumos u'Ma'asros had been taken, it turns out that there was never any issue in the first place.

He goes on to bring proofs to both sides of this discussion.

I hope this helps,

Aharon Steiner