Friday, December 22, 2017

Bitcoin and halacha

The Bitcoin cryptocurrency has recently been in the news as its price continues to escalate. I was lucky enough to hear a friend of mine, a religious computer scientist named A., give a shiur on the halachic aspects of Bitcoin. Here is a summary based on my notes.


Mishna Bava Metzia 4:1 discusses the case of using money to buy goods. It specifies, using several examples, that the kinyan must be done on the goods and not the money. For example, one may do "meshicha" on a utensil, which transfers ownership of the utensil to one party and the money to the other party. The reverse does not work: doing a "kinyan" on the money does not transfer ownership of the utensil.

(The gemara explains that the law was originally different. On a deoraita level, transactions are in fact done via money rather than goods. However, rabbinic law reversed the procedure. This was done to avoid the situation where the money had been transferred, making the transaction valid, but the goods were still with their original owner. Were the goods to be endangered due to a fire or similar disaster, the possessor of the goods would not exert himself to save them, since they no longer belonged to him. Chazal did not like this possibility, so they changed the law to make the transaction dependent on transfer of goods.)

The upshot of these discussions is that one cannot take for granted that one's commercial activity has automatic halachic validity. If you want your transaction to be halachically valid, you must look closely at how you are performing it. So, for example, waving a lulav bought with a credit card might be halachically problematic. To perform the mitzvah on the first day of Sukkot, the lulav must halachically belong to you, but was the credit card transaction valid? Was it you paying the lulav seller, or the credit card company?

Sometimes this type of question is resolved by government regulation - "dina demalchuta dina". But this is not the case with Bitcoin, where the transactions happen online independent of any government, and generally without any government policy on their validity.

In this case, we might turn to "situmta" ("custom of the merchants") - an accepted commercial practice which halacha recognizes, even though it differs from the Torah's laws of transaction.

The key question regarding bitcoin transactions is: at what point does the transaction go through?

Imagine the case of a dishonest person who has $10 in their bank account, and writes $10 checks to two separate people. One of the checks is cashed, and their bank account goes down to $0. From now on, the second check will bounce rather than go through.

This works because there is a centralized authority - the bank - keeping track of how much money people have. However, no such authority exists regarding Bitcoin. You can send money to two different people, even if you don't have it. After some time, one of the transfers will be "confirmed", and the other is not.

In fact, it's possible for BOTH transactions can be confirmed. This causes the transaction record to "fork", as half the network thinks that one transfer went through, and the other half thinks that the other transaction went through. So there are two, mutually contradictory "histories" held by Bitcoin miners on the internet.

What happens in this situation? When the next transaction occurs, the network's nodes will each choose one of the two histories, generally the more popular one. So one of the two histories will come to dominate the network, while the other becomes less and less prevalent until it disappears. This process occurs at an exponential rate, with one history spreading very quickly. Generally, the rule of thumb is that if your transaction is still recognized as valid after 6 other transactions, it is safe to assume that the Bitcoin network has reached equilibrium, and the entire network recognizes it as valid.

But even after 6, or 50, or any number of transactions you can never know for sure that the transaction is valid. There is still an infinitesimally small chance that it will be rejected in the future. From a halachic perspective, this is the interesting bit - there is no real finality to any transaction, unlike "normal" transactions, where an object is physically transferred at a particular moment. At what point, then, does halacha recognize a Bitcoin transaction as a valid kinyan?

It is hard to come up with an objective answer to this. But perhaps the rule of "situmta" applies - the 6-transaction rule is the accepted practice of Bitcoin "merchants", and therefore halacha accepts it as well.


In the "chalifin" kinyan, one party takes a object from the other, and that causes the entire kinyan to be valid. For example, if one is trading a donkey for a cow, one can take possession of the donkey, and ownership of the cow will be transferred at the same moment.

There is a rule (Bava Metzia 45b) that "a coin cannot be used for chalifin", because "its imprint may be nullified" ("tzurta avida devetala"). That is to say, a coin is "fiat currency" which derives its value (primarily) from the government stamp of approval rather than from the metal it is made of. So if the government decides no longer to recognize the coin, its value disappears. The gemara considers this to be mutually incompatible with being the object transferred in chalifin.

(Note: the shiur giver referred me to an article by R' Asher Meir on the Gush website, asserting that in the time of mishna the value of a coin equalled the value of the silver in it, while by the gemara's time a coin's value was already mostly by the fiat of the government. I was not able to find the article offhand, but this is interesting historical context.)

What about Bitcoin? Bitcoin is a distributed currency that no government has the power to devalue. So perhaps it is usable for chalifin?


The Torah prohibits loaning money with interest.

In addition, the rabbis prohibited lending "seah beseah" (one measure of wheat now, in exchange for one measure of wheat in the future), because the price of wheat changes with time, so the value you return may be greater than the value you borrowed, resembling the payment of a loan with interest. Some amoraim extended this prohibition to lending "zahav bezahav" - gold for gold, at a time when money was typically silver.

The exchange rate of Bitcoin fluctuates very quickly, so it would seem that lending Bitcoin and returning an equal amount of Bitcoin would be problematic.


For a Bitcoin transaction to take place, one must provide a cryptographic "signature" to ensure they are the owner of the money they are transferring. There is also a concept called "multisignature", by which multiple signatures must be provided before money can be sent. Alternatively, an account can be set up so that the approval of *either* partner, rather than both, is sufficient to send money.

Just like a physical object can be entrusted to another person for guarding, one can imagine Bitcoins being entrusted to multiple people for guarding. If so, the halachic laws of shmirah might would need to be applied in new ways.


In halacha, lost objects must be returned if they have an identifying sign ("siman") that allows them to be returned to the correct owner.

Bitcoin value is accessed by a "private key", a unique randomly generated string of numbers. If you forget the number, your value is lost forever, since with current computing resources it's essentially impossible to regenerate the string.

Since this number is so unique, one could imagine that it is the best type of "siman" imaginable.

However, in the case of "avdah mimenu umikol adam" - when it seems essentially impossible for any person to recover the object, for example if it was washed away by a river - there is no requirement to return the object, even if it has a siman.

Since it is so difficult to recover a lost Bitcoin signature, one might think that it falls in this category, of objects that do not need to be returned if they are somehow found.

Saturday, December 02, 2017

Moreh Nevuchim

I recently finished reading Moreh Nevuchim. Here are my notes, organized by page number in the Friedlander translation. If you want to look up an issue, which is not directly referred to in Friedlander's chapter titles, this may be a useful guide.

44 Revelation is an inferior way of communicating what those who are capable should learn through philosophy (also 394)
78 Change implies imperfection either before or after. God as perfect being cannot have changed. Thus can only act on others, not be acted upon
84 Whenever you prove God doesn't have a certain quality, you become more perfect. The more qualities, the higher level you are relative to other people
95 YKVK=existence
99 One can use an interpretation which solves a problem, even if it violates certain grammar rules
104 God is the (Platonic) form of forms
130 Earth is round (also 277)
139 Omnipotence doesn't mean that God can do impossibilities
155 Purpose of the book
178 No-proof is better than a wrong proof
184 Proof from design and diversity (also 291)
195 Being moral helps you reason correctly
196 Directed to a specific person?
198 Aristotle's system is correct refarding the earth, but not the sky/spheres
199 Eternal universe contradicts Tanach less than does incorporeality and other important ideas
210 Miracles built into nature
212 Time created as part of creation
216 Secrets
218 Language is conventional, not natural
228 Intellect leads to logic and imagination. Prophet=both. Intellect vs senses. Teaching is better than learning
232 Torah is not painful, but seems cruel to the carnal
239 Anagrams in Tanach
248 Long lives in Breishit
250 Providence and free will
257 Disagrees with Onkelus re Yehezkel
261 Yetzer hatov <= platonic forms. Yetzer hara <= physical substance
264 Hebrew doesn't have words for bodily functions
265 Physicality = wall between us and God
266 Evil = absence of good. (Its cause: absence of wisdom)
297 Interpretation of Job excites Rambam
310 Only difference between philosophers and us is eternal universe
353 Midrash
356 Belief in God, angels, prophecy, law in that order
375 Torah, like nature, is only partly comprehensible - but that part is sufficient to give us a sense of wonder
385 True worship of God means thinking deeply about the idea of God
386 love of God=knowledge of God
387 intellectual worship of God, not "hollow emotions"
392 Loving God comes from theology, fear from mitzvah performance

Saturday, July 15, 2017

The well story

Parshat Chukat contains a short and cryptic story about a well:
...from there to the well, which is the well about which Hashem said to Moshe, "Gather the people, and I will give them water." Then Israel sang this song: "Rise up, O well - answer it - A well dug by princes, carved by the leaders of the people, with the staff, with their rods" (Bamidbar 21:16-18)
Even in this short passage, we see apparent inconsistencies.
  • First, God says that He will provide the people with water.
  • Then, the people say "Rise up, O well", which perhaps sounds like a well is rising miraculously, as God might do in these circumstances.
  • However, then the people say that the well was dug by human beings, not by God!
  • Then they say that the digging was done by the leaders (not normal manual laborers), and it used their staffs (not normal digging tools)!
How are these pieces to be put together into a single coherent story?

I think the answer is to look at how miracles were often performed in earlier events in the Torah. In Egypt, for example, the norm was for Moshe to point his staff at something, or hit something with his staff, and a miracle would then occur involving that something.

What is the parallel here? God announced to Moshe that God would provide water. Moshe and Aharon got their staffs, and pointed at or hit a certain rock. This caused water to miraculously come from the ground. At which point the people sung their song.

This explains how all the events could logically have happened. But there is still one inconsistency. Initially God announced that He would provide water. But in the song, the people do not mention God's role, but only the role of humans who "dug" the well. Why the difference?

To answer this, let's examine a very similar story from just one chapter earlier. At Mei Merivah, Moshe was commanded to take his staff and provide water for the people. It's not clear exactly why, but God saw Moshe's ensuing actions as a failure to "sanctify God" before the people. One common interpretation is that Moshe hit the rock, rather than speaking to it, which indicated that Moshe rather than God was the one providing water.

If so, at the time of our story with the well, the people might still have believe that Moshe not God was providing them with water. Here Moshe pointed his staff at the ground and water rose from it, which should have been a sign of God's miraculous intervention. But the people mistakenly saw it as an act of Moshe. And when they sang their song, cheering the rise of the well, they chose to praise Moshe and Aharon rather than God.

This understanding resolves all the loose ends in the story. How does it fit with our understanding of the rest of the Torah?

The most obvious question is about "song". There is a midrashic idea (appearing in the Mechilta, Tanchuma, the beginning of Targum Shir Hashirim, and other sources, generally searchable on Bar Ilan CD with the words "eser shirot") which lists ten "songs" that have been sung throughout history. The exact list varies slightly between sources, but here is a common version: in Egypt; after the splitting of the sea; at the well (our story); around parshat Haazinu; by Yehoshua; by Devorah and Barak; by David; by Shlomo; by Shlomo in Shir Hashirim; and the final song which will be recited in the future messianic era. The common element in these "songs" is that they are Divinely inspired, righteous, and holy. But according to my interpretation of the well song, rather than being holy, it was based on a massive theological mistake. It seems that my explanation of the story is incompatible with this view of "song". Accepting this midrash is not an article of faith, but we should be clear what we would have to give up by accepting my explanation.

The second question is the place of the story in Sefer Bamidbar. It is commonly accepted that the Jewish people's behavior was different in the second and fourtieth years of their desert journey. The second year was full of rebellions, culminating in the stories of the spies and Korach. The fourtieth year, in contrast, showed a Jewish people that had learned their lessons, and were now spiritually ready to enter the land of Israel.

But this picture is not so clear. The story of the snakes (21:4-9) was yet another rebellion ending in punishment (though for several reasons it might be more positive than the previous rebellions). And the episode with the daughters of Moav was so serious that God raised the possibility of destroying the people entirely (25:11). Of course there are also more positive stories, such as the people making and keeping a vow to God (21:1-3). So I think there is enough room here for another negative story. The people was still quite flawed, but after 40 years in the desert, they were sufficiently improved that they could handle conquering and living in the land of Israel.

Saturday, January 21, 2017

Thoughts on Shemot

Aharon your brother... he comes out to meet you, and he will see you and be happy in his heart. (Shemot 4:14)

Why does it matter that Aharon was "happy in his heart"?

Aharon was the oldest, firstborn brother, and here he was called upon to take a subordinate role to Moshe. Elsewhere in the Torah, older brothers typically get angry and jealous when their firstborn role is taken away from them.

The exile in Egypt started when Yosef's brothers sold him into slavery rather than accepting a subordinate role to him. Fittingly, it ended when Aharon was able to graciously and peacefully accept that Moshe had been chosen as the leader, and work together with Moshe to get the Israelites freed.