Underneath the Temple Mount in Jerusalem, according to an old Jewish legend, there is a secret cave. Inside the secret cave, there is a hidden chamber, and in the chamber sits the Ark of the Covenant, lost since the time of the Bible. Inside the ark, unseen for thousands of years, are the tablets that Moses brought down from Mount Sinai. Unseen tablets in a lost ark in a hidden chamber in a secret cave underneath a vanished temple.But no one needs to see the tablets themselves because we know what is on them: The Ten Commandments. Do not murder. Do not covet your neighbor’s wife. What the legend of the hidden tablets reminds us is that the most secret things are often the most obvious. The nistar, Judaism’s secret teachings, are the same as the nigleh, the revealed teachings.There are five types of secrets, each with its own moral qualities: secret thoughts, secret languages, secret powers, secret books, and secret truths. All of these are obvious and well known, even though they are secret.Secret thoughts: The fact that our thoughts are intrinsically secret, that no one else can listen to them unless we say them out loud, is intrinsic to our sense of being individuals. No one else has our memories, no one shares our emotions, and no one knows our hidden thoughts. Because this secrecy is such a large part of being an individual, it is morally incumbent on us to respect the secret thoughts of others. However well we may think we know other people, and however transparent their thoughts may seem to us, there is a core of secret thoughts that each individual protects.In the Bible, the story of Joseph begins when, at first, he reveals his secret dreams to his brothers, which causes them to hate him. Later, when he is ruler of Egypt, he hides his thoughts from them, and the results are much happier. He eventually reveals his identity to his brothers, but he never reveals his hidden thoughts to them.Now, the truth is, as psychoanalysts and laymen both discover, our closely guarded secrets are only rarely very different from anyone else’s; they are often commonplace and even banal (“Rosebud”). Many of Joseph’s secret thoughts about his brothers and about what they did to him are not particularly hard to guess, and one imagines that his brothers had some sort of inkling about what Joseph was thinking. Still, the brothers do not ask, and Joseph doesn’t tell them. Banal they may be, but our secret thoughts are ours—more than that, they are us—and on that account, they are precious.Secret Languages: Later in the Bible, there is a story that is the opposite of Joseph’s: the story of David and Jonathan. The two of them arrange a secret meeting and a secret code known only to them: If Jonathan shoots his arrows far away, it means that David is in danger; if he shoots them closer in, it is safe. Not even Jonathan’s servant is in on their secret. Every intimate relationship creates its own shared language—this is the teaching of this odd little story, which is read in synagogue on Sabbaths that fall on the day before Rosh Chodesh. A secret language is a second type of secret, but it’s the opposite of the first because by definition it is shared.The Ten Commandments have both aspects. In one respect, they are the most famous verses in the Torah, shared with all humanity, Israel’s love letter to the world. In another respect, they are not the Ten Commandments at all, but the עשרת הדיברות, written in Hebrew, the secret language of the Jews, or the secret language between the Jews and God. They are the secret thoughts of the Jews, which are not different or not very different from the basic moral principles of other groups.Powerful Secrets: The world is, to a large extent, not only unknown to each of us but unknowable. To know even a little of it is a powerful thing, and so every secret is potentially a secret power. Among Bible stories, the story of Samson is based on this truth; in fact, the entire Book of Judges is full of stories involving secrets and their powers.A few chapters later, toward the beginning of the Book of Samuel, the Bible discusses whether the Ark of the Covenant is a sort of secret weapon that the Israelites can use to win their wars. It isn’t. The Israelites take the ark into battle, but the Philistines win the battle anyway. The Bible harps on this truth. The only “secret weapons” that the Bible recognizes are doing justice and being kind, having faith, and fearing God. Proverbs and Ecclesiastes add common prudence to the biblical armory. These weapons are secret in the sense of being spiritual or internal, compared to the physical and external weapons, the chariots and armor, whose value the Bible nearly always mocks.Secret Knowledge: Secret knowledge is often lost, even usually lost. Someone knows something once, but then it is forgotten. However, if the person who knows it has taken care to write it down, it can be recovered.The famous example of secret knowledge in the Bible is the Book of Memories in the story of Esther, which reminds King Ahasuerus that Mordechai the Jew once saved him from a murder plot. Yet while books are very useful for remembering secret knowledge, they can also get lost. The lost Book of Covenant is discovered by the Temple priests when they are doing some repairs, and they bring it to King Josiah, who leads the people in a wave of repentance. The Torah is constantly being lost or forgotten and constantly being recovered: hence the legend of the hidden Ark.Secret Truths: Often knowledge is lost not by forgetting or losing something but by creating beliefs that aren’t true. On this account, there is a fifth kind of secret: a secret truth.There are many reasons why people believe things that are false. Maimonides discusses this question carefully in the introduction to The Guide for the Perplexed and again in many places throughout the book. Sometimes we deceive one another, and sometimes we deceive ourselves. Sometimes half-truths or half-forgotten truths engender falsehoods. Often our attempts to learn things fall short, and we mistakenly become convinced of beliefs that are false, which then crystallize into orthodoxies and dogmas.For all of these reasons, the process of approaching the truth as presented by Maimonides is as much a process of gradual skepticism as it is a process of gradual discovery. Maimonides famously applies this insight to the knowledge of God, the so-called via negativa. Our knowledge increases (he claims) as we learn more and more deeply what God is not, and as we become more and more skeptical of the literal truth of claims about God.The same logic applies to many types of knowledge. Often it can be difficult—sometimes it requires great courage—to hold onto basic truths about the world. Two and two is four. Summer is hot. The stars are very high up and very far away. We have to brush aside powerful voices that tell us the opposite.This is true of moral knowledge as well. The Torah is complex; there are 613 commandments, but the basic moral truths are very simple. Don’t kill people. Don’t swear false oaths. Don’t covet your neighbor’s wife or his house. Yet cultures are easily led into thinking that killing certain types of people is okay or that it is virtuous to tell lies in defense of some ostensibly higher truth.It is on this account that it makes sense to think of the Ten Commandments as being hidden or lost. Everyone knows what the Commandments say, and there are copies available in every language on the planet; yet they are also unseen tablets in a closed box in a hidden chamber inside a secret cave, waiting to be discovered.