Gemara

(Mishnah): We do not expound Ma'ase Bereishis to two (or more) Talmidim (at a time).

(Beraisa): "She'al Na l'Yamim Rishonim" - an individual may ask, but two may not.

13a (Mishnah): We do not expound Ma'ase Merkavah even to one (unless he is a Chacham who understands by himself).

(R. Chiya - Beraisa): We do teach him the Roshei Perakim (beginnings of the chapters).

Version #1 (R. Zeira): We teach Roshei Perakim only to an Av Beis Din and to anyone with fear in his heart (he will not be lightheaded).

Version #2 (R. Zeira): We teach Roshei Perakim to an Av Beis Din only if there is fear in his heart.

(R. Ami): We teach secrets of Torah only to one with five qualifications: he is "a judge over 50 people, esteemed, a counselor, Chacham Charashim (others have nothing to say when he speaks in Torah), and Navon Lachash (understands Sisrei Torah, which must be said quietly; alternatively, he knows how to conceal secrets concisely).

R. Yochanan offered to teach Ma'ase Merkavah to R. Elazar. R. Elazar declined, because he was not old.

Rav Yosef knew Ma'ase Merkavah; the elders of Pumbedisa knew Ma'ase Bereishis. They asked him to teach to them Ma'ase Merkavah; he asked them to teach to him Ma'ase Bereishis. After they did, they asked him to teach to them Ma'ase Merkavah.

Rav Yosef: It says "Devash v'Chalav Tachas Leshonech" - things sweeter than honey and milk should be under your tongue (do not reveal secrets of Torah).

(R. Avahu): We learn from "Kevasim li'Lvushecha" - things in the Kivshan (hidden places) of the world should be under your garment (do not reveal them).

The elders: We already learned up to "Va'Yomer Elai Ben Adam"!

Rav Yosef: You have already learned what Chachamim were concerned about (the verses that discuss the form and appearance of the Shechinah).

Rishonim

Rambam (Hilchos Yesodei ha'Torah 2:12): What I explained in Perakim 1-2 (names of Hash-m and angels, anthropomorphisms of Hash-m, His existence and knowledge, visions of angels...) is a drop in the sea of what there is to explain about Ma'ase Merkavah. Early Chachamim commanded to expound these matters only to one person at a time. He must be a Chacham who understands the depth of a matter by himself when the introduction and a small amount are explained to him.

Magid Mishneh: If one seeks to know such matters without proper preparation contort his thinking and blind his eyes and lead him to sin. He will be unable to understand even what he knew beforehand.

Rambam (4:10,11): What I explained in the Perakim 3-4 (heavenly bodies, the elements, forms...) is a drop in the bucket of what there is to explain about Ma'ase Bereishis. They are deep, but not as deep as Ma'ase Merkavah. Early Chachamim commanded to expound these matters only to one person at a time. He need not be able to understand things by himself. We inform him everything he can know about these matters. We do not expound them in public because not everyone can understand them clearly.

Magid Mishneh: The former are called Ma'ase Merkavah because this is how they appeared to Yechezkeil. The latter are called Ma'ase Bereishis because the Torah describes their creation. Everyone can understand them because they are physical.

Rambam (13): Perakim 1-4 are called Pardes. Four Chachamim entered the Pardes, and not all of these were able to understand the matters clearly. One should not enter until he knows what is forbidden and permitted and all Mitzvos. These (Isur v'Heter and Mitzvos) are the proper introduction. They straighten a person's mind; they are the great good that Hash-m bestowed to settle this world and enable people to inherit the world to come. Everyone can understand them, including women and people of weak intellect.

Rambam (Hilchos Talmud Torah 1:12): Matters called 'Pardes' are included in Talmud.

Question (Drishah YD 246:2): It seems that Pardes is Chachmah of nature. This is not included in Gemara! The Rambam must refer to what is above and below, i.e. investigation of the Divine.

Rambam (introduction to Mishnah, DH veha'Revi'i): Do not suspect that Chachamim hid secrets of Torah from each other due to lowly Midos of stinginess or a desire to aggrandize themselves. Rather, it was when they perceived that the other was not fitting for them. It is impossible to teach such matters or expound them in public. We make hidden hints to them in stories. If the listener has prepared himself and Hash-m desires him, Hash-m removes the veil from the listener's heart and he understands according to his intellect. One does not strive to understand them like with other Divrei Torah. Rather, he directs his heart to Hash-m and prays and requests that He reveal to him secrets hidden in Kisei ha'Kodesh - "Gal Einai v'Abitah Nifla'os mi'Torasecha". When Hash-m reveals secrets to a person, he must cover them up. He may hint about them only to someone at least as learned as himself. One may teach to the Rabim only through parables and riddles. When the listeners mature, they will understand "Mashal u'Melitzah Divrei Chachamim v'Chidosam".

Moreh Nevuchim (1:33): Secrets of Torah are like bread, meat and wine. They are good for the body, but we do not feed them to a newborn because he is not ready for them. This is why Chachamim hid them and the Torah itself speaks like people do. One who is Shalem will understand the hidden meaning.

Moreh Nevuchim (34,35): Mishlei discusses a lazy person who does not want to learn. This is a parable for one who desires to know Sisrei Torah without toiling to understand the prerequisites. His desire ruins him, like one who heads to a destination and falls into a pit from which he cannot escape. It would have been better had he stayed home. One who is not Shalem in Midos, e.g. one whose nature is to get angry, cannot attain Sisrei Torah, even if he accustoms himself to act properly. As long as a bad Midah flares inside him, teaching him Sisrei Torah is folly. This is why we do not teach it to youths. We do not teach it to one who is too involved with a wife or children or with physical pleasures. It is proper to teach everyone that Hash-m is non-physical, incomparable and unchanging.

Ramban (beginning and end of introduction to Bereishis): Every deep secret and Chachmah is hidden in the Torah... One should not contemplate to understand the hints I bring about secrets of Torah. This will not lead to understanding them, only to error, folly and great damage that cannot be forgiven. Hash-m will show wonders to whom He desires. Do not seek things beyond you. They can be attained only from the mouth of a Chacham Mekubal to the ear of an understanding receiver. Do not engage in what is beyond you.

Poskim

Rema (YD 246:4): One should learn only Mikra, Mishnah, Gemara and Poskim based on them. Through this one acquires this world and the next. One should not learn other Chachmos. One may learn other Chachmos on occasion, as long as it is not Sifrei Minim. Chachamim called this 'strolling in the Pardes'. One may do so only after he has filled his stomach with meat and wine (Shas and Poskim) and knows Isur v'Heter and laws of the Mitzvos.

Shach (6): The 'other Chachmos' are things like philosophy, astronomy and Kabalah. The Rema is not precise. The Rambam says that only Ma'ase Bereishis and Ma'ase Merkavah are called Pardes. Mekubalim and Acharonim stress that one may not learn Kabalah before he fills his stmach with Shas. Some say that he must be at least 40, like it says (Avos 5:26) that 40 is the age for Binah. One also needs Kedushah, Taharah, Zerizus, and Nekiyus. Most who brazenly learned it before they were ready died prematurely.

Gra (18): The four who entered the Pardes were young, except for R. Akiva. Therefore, they died. R. Elazar refused to learn because he was young.

Chavos Ya'ir (210 DH v'Zeh): Shemen ha'Ma'or says that one who did not receive Sisrei Torah from a Rebbi and thinks to understand them by himself transgresses "Lo Ta'aseh Lecha Pesel". Toras ha'Olah says that he has not seen one who truly learned Kabalah from a proper Rebbi, yet everyone jumps to learn it, especially nowadays that Zohar and other Sifrei Kabalah have been published. People who do not even understand Rashi are attracted to secrets that they are totally incapable of understanding. They explain it according to their understanding, which cannot be correct because it is not from a Rebbi. One who knows little and expounds such matters in public will be held accountable for it.

Chacham Tzvi (36 DH Teshuvah): Even if the Zohar is the opposite of the Poskim, we do not abandon the Poskim due to Sisrei Torah. We rely on the Zohar only for something not mentioned in the Gemara or Poskim, or when there is an argument among the Poskim. Everything in the Zohar is concealed in 1000 coverings. Who can say that he understands the intent?!

Yechavah Da'as (4:47 DH Emnam and DH Ulam): The value of learning Kabalah is awesome. The Zohar also describes the punishment for one who does not engage in secrets of Torah. After death, Hash-m asks one who learned Talmud 'Did you look at My Merkavah? I have Hana'ah only when Chachamim engage in the Torah and look at the Merkavah, how My Kisei ha'Kavod stands, how the Chashmalim stand. This is My greatness, that My children recognize My honor!' However, this is only for a Chacham full of Yir'as Hash-m who has filled his stomach with Shas and Poskim b'Iyun and learns clearly and according to Halachah. The Maharsha tirades against unfitting people who learn it. They are Megaleh Panim b'Torah she'Lo k'Halachah; people must protest against them. One who learns must know that the Rebbi is a Mekubal who had the proper preparations.

Aruch ha'Shulchan (15): Happy is one who merits to learn Kabalah. It enables one to understand the depth of Torah! However, one needs Kedushah, Taharah... most who learned it prematurely died prematurely. It is proper to learn from the mouth of a Chacham Mekubal. Unfortunately, we lack this. Instead, we have Seforim that guide to the beginning of learning Kabalah, i.e. Shefa Tal, Sha'arei Orah, and Avodas ha'Kodesh.