In the מסכת שביעית פרק שני we have a question as to when is the last time that one is permitted to plow a field before שמיטה. The תנא קמא says: עד שתכלה הלחה כל זמן שבני אדם חורשים לטע במקשאות ובמדלעות. This bothers רבי שמעון who says: נתת תורת כל אחד ואחד בידו. The תנא קמא does not set a definite date and therefore leaves the date somewhat subjective.
As I have read through the Mishnah I have been struck by two things:
a) the focus on שיעורים. A large proportion of the Mishnah is dedicated to the exact dates, times, measures etc. that are required or prohibited or permitted for various מצוות.
b) as the very beginning of the Talmud Bavli points out at the beginning of ברכות, the Mishnah does not usually bother to give much context. It jumps straight into these details of שיעורים. I find it almost humorous that the Gemara in ברכות begins with תנא היכא קאי – as though it was unfamiliar with the Mishnah which often (but not always) jumps into topics with no introduction and assumes that the “reader” has a general familiarity with the subject matter.
My thought is then, is the Mishnah – at least in part – a discussion of שיעורים? Everyone knows the basic laws and traditions, what they might not know are the details, and is this perhaps one of the motivations for the Mishnah – to delineate the boundaries of all of those laws and traditions?