Avos chapter 3, Mishnah 10 (in some versions it is numbered as Mishnah 8). The opening of the Mishnah is unusual: "Rabbi Dosa b'Rabbi Yannai mishum Rabbi Meir omer" - here we do not have a sage transmitting a teaching in his own name, but a sage quoting another sage, and it is doubtful whether there are other examples of this in the tractate. The wording "mishum Rabbi Meir" (in the name of Rabbi Meir) indicates that he did not hear the teaching from Rabbi Meir himself, but rather heard the idea attributed to Rabbi Meir, and repeated it.
Rabbi Dosa b'Rabbi Yannai is among the very last of the Tannaim. His father, Yannai, was a student of Rabbi Yehudah HaNasi, the redactor of the Mishnah, so he is the son of a student of the last of the Tannaim. And the subject of his teaching is: the importance of reviewing one's learning, and not forgetting one's Torah.
The words of the Mishnah:
"Kol hashocheiach davar echad mimishnaso, ma'aleh alav haKasuv k'ilu mischayeiv b'nafsho" - anyone who forgets even one thing from his learning, Hashem regards him as though he were guilty of taking his own life and responsible for his own soul. "Shene'emar: rak hishamer lecha ushmor nafshecha me'od pen tishkach es hadevarim asher ra'u einecha" - "as it says: only guard yourself and guard your soul exceedingly, lest you forget the things your eyes have seen."
In its original context, the verse refers to the command not to forget the revelation at Mount Sinai - the experience Israel had as witnesses to the revelation of the Shechinah and to the confirmation that Moshe Rabbeinu is a prophet of Hashem. Here, however, the verse is understood to mean that one should not forget the things he has heard, not literally at Sinai but even afterward, from the things that were transmitted at Sinai, that is, Torah.
"Yachol afilu takfah alav mishnaso?" - one might have thought that this applies even to someone who forgot because his learning was simply too hard for him, whether to grasp or to retain, or because of distractions competing for his attention, and the forgetting was not his fault. "Talmud lomar: u'fen yasuru milvavcha kol yemei chayecha" - "the verse teaches: and lest they be removed from your heart all the days of your life." The word "yasuru" indicates an active removal, and from here we learn that "ein hu mischayeiv b'nafsho ad sheyeisheiv v'yesirem milibo" - he is not guilty regarding his soul unless he deliberately removes the words from his heart.
And how does a person remove things from his heart and his mind? Through action, or through what is equivalent to it: through negligence and carelessness. One who is grossly negligent and makes no effort at all, when he is able to, to ensure that he will not forget his learning - he is responsible for the loss of his Torah, and the risk is very high.
And what does "guilty regarding his soul" mean? There are two approaches to explaining it:
Some understand that Torah protects a person and guards his life, and one who neglects his Torah loses that protection.
Others understand it more generally: that the very thing itself - actively allowing oneself to forget one's learning - is a kind of crime in its own right.
"One thing" versus "the things":
The meaning of the Mishnah is straightforward, and the main point of precision lies in its wording: the Mishnah says "kol hashocheiach davar echad" - one single thing, whereas the verse brought as its source is stated in the plural: "lest you forget the things." How does the verse yield liability even for a single thing?
One answer is that the entire Torah is a single connected system, and once even one part of it is lost, the whole begins to come apart. This is reflected in the laws of a Sefer Torah: one missing letter renders the whole thing invalid, and the same is true of tefillin and of a mezuzah. So too here: if one law is missing from his mind, everything is lacking, and it is not merely one thing that is missing, for many things are bound up in that loss, and it turns out that he has lost many things.
And why is the punishment so severe? "Forfeits his life" is an extreme expression. Some commentators explained that one who forgets is liable to rule mistakenly and reach a wrong decision because he has forgotten the halachos, and that is a bad outcome. Others say that it does not depend on this at all: not because he will err in ruling as a judge or as a halachic authority, and not even as one who instructs in his own home, but rather the very fact that he forgot what he learned.
Two mitzvos: Torah study and knowledge of Torah:
Here a very important point emerges, which in my view is the foundation on which this Mishnah stands: there is indeed a mitzvah of Torah study, and perhaps Torah study is equal to all the rest. But there is a second requirement, and it is a separate mitzvah - knowing the Torah. Learning that ends in forgetting is not enough; a person must learn with the intention of remembering, and build in his mind a picture of the entire Torah.
And in practice, every Jew is expected to learn the whole Torah. True, "it is not upon you to complete the work" - he cannot finish it, and he must do the best he can - but it is worth grasping what this means.
The three parts of Torah according to the Rambam:
The Rambam in Hilchos Talmud Torah divides the Torah into three parts:
Written Torah - the twenty-four books of Tanach.
Oral Torah - the principles that Moshe received at Har Sinai, and also the words of the Sages that expanded out of them and afterward. The Rambam himself says that this is essentially what he assembled in his Mishneh Torah; and for simplicity's sake we can say: the content of the Mishnayos.
Talmud - the Gemara portion, the elaboration of the principles, their application, their explanation and their expansion.
The first part is of course finite - these are the twenty-four books and no more. The second part is also a large quantity of material, but entirely finite: the Mishnayos number about four thousand two hundred and something, and someone who learns two Mishnayos a day finishes them all in about seven years; and the Rambam, large as it is, can be finished in three years by learning one chapter a day. These are big projects, but they are not that big. With a concentrated effort - sitting down and learning a chapter of Rambam every day, and working to review and remember it - it is doable.
The third part, by contrast, the Talmud portion, is the elaboration of the principles and their application, their explanation and their expansion, and therefore it goes on endlessly. But the basic material is finite.
The Gemara in Kiddushin says: "A person should always learn Torah and afterward marry a wife," and the Rambam rules this way in halachah - first a person should learn Torah and then marry. The Rosh asks his famous question: when will a person ever finish learning Torah so that he can marry? According to our approach the answer is simple: in the Rambam's introduction to Hilchos Talmud Torah it is explained that a person must divide his day into three parts - and this is an explicit Gemara - to know the Written Torah with its verses, and to know the Oral Torah, which is the principles, and then the entire Torah is at his command. The rest, the third part, is the endless part that one can never reach the end of, and one cannot wait for it in order to marry; but one can marry once he has mastered the entire Written Torah and the Oral Torah as the Rambam understands it, namely the principles.
It is worth noting that this was brought as halachah in the Shulchan Aruch, and the poskim ruled exactly like the Rambam - a division into three parts. And it is known that Tosafos disagree and hold that it is not so.
Knowledge of Torah as a goal in its own right:
The point that needs clarifying, and it is the fundamental point underlying our Mishnah, is that knowing Torah must be a goal in its own right. It is not enough for a person to be a faithful Daf Yomi learner or a steady learner of Mishnayos - two mishnayos a day, a page every day. That is good, that is wonderful, it is a mitzvah, and Torah study is a wonderful thing. But one can practically guarantee that in seven and a half years, when that same learner comes back to today's daf, he will not remember it at all, and the page will look completely new to him. One who learns something and does not review it - his learning has effectively been lost. And since the outcome is foreseeable, this is negligence: it is obvious that the learning will be lost, and this is not the way of a careful guardian, nor is it a responsible approach.
The Secret of Review:
Therefore our Mishnah teaches that a person must build into his Torah study a system for reviewing what he has learned. And the basic secret of review is this: review before you forget. That is what makes it difficult, since it is boring to review things you already know; but one who reviews before forgetting - the learning takes hold in him and is not forgotten. Review, wait a day, then two days, then four days; or review once this week, again the next day, again next week, next month, next year - whatever system you choose. The main thing is to keep reviewing before you forget, so that everything stays in place. That is how knowledge accumulates, and in the end a person knows enormous amounts of material.
And if the objection is: I will never reach the end, there is so much to learn, and if I spend so much time on review I will never move forward - the answer is "lo alecha hamelachah ligmor", it is not upon you to complete the work, and a person can only do what is within his ability. But to learn with the clear knowledge that the learning is destined to be forgotten - that is a crime, "mischayev benafsho". Do not do that. Invest great effort not only in the learning itself, but also in remembering what you have learned.