The overall subject of the book is the way in which Jewish law addresses doubts, either as to the facts or as to the applicable law. The Talmud provides a series of presumptions, in favour of strictness or leniency depending on the circumstances. One series of questions concern the way in which these presumptions interact. Another is the more fundamental question of how presumptions work. That is, does a presumption have the effect of assimilating the doubtful cases to the certain cases in all respects, or are doubtful cases a third category with its own special laws, alongside the certainly included and the certainly excluded?