Postel’s Principle as Moral Aphorism

[All the usual dis­claimers. Wan­ders dan­ger­ously close to moral rel­a­tivism. Cross-posted from Grand, Unified, Empty.]


Pos­tel’s Prin­ci­ple (also known as the Ro­bust­ness Prin­ci­ple) is an ob­scure lit­tle guideline some­what pop­u­lar among com­puter pro­gram­mers, in par­tic­u­larly those work­ing on net­work pro­to­cols. The origi­nal goes like this:

Be con­ser­va­tive in what you do, be liberal in what you ac­cept from oth­ers.

My par­ents were both com­puter pro­gram­mers, as am I, and my first job as a pro­gram­mer was work­ing on net­work pro­to­cols, so it shouldn’t be too sur­pris­ing that I ran across this prin­ci­ple a long, long time ago. I sus­pect I heard it while still a teenager, be­fore finish­ing high school, but I hon­estly don’t re­mem­ber. Suffice to say that it’s been kick­ing around my brain for a long time.

As a rule of thumb in com­puter pro­gram­ming, Pos­tel’s Prin­ci­ple has some ba­sic ad­van­tages. You should be con­ser­va­tive in what you do be­cause pro­duc­ing out­put that isn’t strictly com­pli­ant with the speci­fi­ca­tion risks other pro­grams be­ing un­able to read your data. Con­versely, you should be liberal in what you ac­cept be­cause other pro­grams might oc­ca­sion­ally pro­duce non-com­pli­ant data, and ideally your pro­gram should be ro­bust and keep work­ing in the face of data that isn’t quite 100% right.

While in re­cent years the long-term effects of Pos­tel’s Prin­ci­ple on soft­ware ecosys­tems have led to some push­back, I’m more in­ter­ested in the fact that Pos­tel’s Prin­ci­ple seems to ap­ply as well just as well as a moral apho­rism as it does in pro­gram­ming. Con­text mat­ters a lot when read­ing, so here’s a list of other apho­risms and pop­u­lar moral phrases to get your brain in the right frame:

  • What would Je­sus do?

  • Ac­tions speak louder than words.

  • If you can’t say some­thing nice, don’t say any­thing at all.

  • Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a life­time.

  • Be con­ser­va­tive in what you do, and liberal in what you ex­pect from oth­ers.


I am, by na­ture, a fairly con­ser­va­tive per­son. I’m also, whether by na­ture or past ex­pe­rience, some­what so­cially sub­or­di­nate; I’m usu­ally much hap­pier in a sec­ondary po­si­tion than in any role of real au­thor­ity, and my self-image tends to be fairly frag­ile. The manosphere would hap­pily write me off as a “beta male”, and I’m sure Jor­dan Peter­son would have some­thing weird to say about lob­sters and sero­tonin.

This com­bi­na­tion of per­son­al­ity traits makes Pos­tel’s Prin­ci­ple a nat­u­ral fit for defin­ing my own be­havi­our. Rather than try­ing to se­ri­ously en­force my own wor­ld­view or ar­gue ag­gres­sively for my own prefer­ences, I en­deav­our not to make waves. The more peo­ple who like me, the more se­cure my situ­a­tion, and the surest way to get peo­ple to like me is to fol­low Pos­tel’s Prin­ci­ple: be con­ser­va­tive in my own ac­tions (or else I might do some­thing they dis­ap­prove of or dis­like), and be liberal in what I ac­cept from oth­ers (be­ing judge­men­tal is a sure way to lose friends).

[Peo­ple who know me IRL will point out that in fact I am pretty judge­men­tal a lot of the time. But I try and re­strict my judg­i­ness (judg­men­tal­ity? judge­men­tal­ism?) to mat­ters of ob­jec­tive effi­ciency, where em­piri­cal re­al­ity will back me up, and avoid any kind of value-based judge­ment. E.g. I will judge you for be­ing an in­effec­tive, in­con­sis­tent fem­i­nist, but never for hold­ing or not hold­ing fem­i­nist val­ues.]

Un­for­tu­nately, of course, the world is a mind-bog­gling huge place with an an­noy­ingly large num­ber of peo­ple, each of whom has their own slightly differ­ent set of moral in­tu­itions. There is clearly no set of be­havi­ours I could perform that will satisfy all of them, so I fo­cus on ap­ply­ing Pos­tel’s Prin­ci­ple to the much smaller set of peo­ple who are in my “so­cial bub­ble” (in the pre-COVID sense). If I’m not likely to in­ter­act with you soon, or on a reg­u­lar ba­sis, then I’m rel­a­tively free to ig­nore your opinion.

Talk­ing about the “set” of peo­ple on whom to ap­ply Pos­tel’s Prin­ci­ple pro­vides a nice segue into the for­mal defi­ni­tions that are im­plicit in the English apho­rism. For my own be­havi­our, it makes sense to think of it like the in­ter­sec­tion op­er­a­tion in set the­ory, or the uni­ver­sal quan­tifier in pred­i­cate logic: some­thing is only morally per­mis­si­ble for me if it is per­mis­si­ble for all of the peo­ple I am likely to in­ter­act with reg­u­larly. Con­versely, of course, the val­ues I must ac­cept with­out judg­ment are the union of the val­ues of the peo­ple I know; it is morally per­mis­si­ble if it is per­mis­si­ble for any of the peo­ple I am likely to in­ter­act with reg­u­larly.


Since the set of ac­tions that are con­sid­ered morally per­mis­si­ble for me are defined effec­tively by my so­cial cir­cle, it be­comes of some im­por­tance to in­ten­tion­ally man­age my so­cial cir­cle. It would be un­ten­able to make such differ­ent friends and col­leagues that the in­ter­sec­tion of their ac­cept­able ac­tions shrinks to noth­ing. In that situ­a­tion I would be forced to make a choice (since in­ac­tion is of course its own kind of ac­tion) and jet­ti­son one group of friends in or­der to open up be­havi­oural ma­noeu­vring space again.

Un­for­tu­nately, it some­times hap­pens that peo­ple change their moral stances, es­pe­cially when un­der pres­sure from other peo­ple who I may not be in­ter­act­ing with di­rectly. Even if I have a sta­ble so­cial cir­cle and be­havi­oural ma­noeu­vring space to­day, to­mor­row one of my friends could de­cide they’re sud­denly a rad­i­cal Is­lamist and force me with a choice. While in some sense “difficult”, many of these choices end up be­ing rather easy; I have no in­ter­est in rad­i­cal Is­lam, and so ul­ti­mately how close I was to this friend rel­a­tive to the rest of my so­cial cir­cle mat­ters only in the very ex­treme case where they were liter­ally my only ac­quain­tance worth speak­ing of.

Again un­for­tu­nately, it some­times hap­pens that large groups of peo­ple change their moral stances all at once. Memes spread in­cred­ibly fast, and a small un­der­cur­rent of change can rapidly be­come a tor­rent when one per­son in a po­si­tion of power or sta­tus chooses a side. This sort of situ­a­tion also forces me with a choice, and of­ten a much more difficult one. Apart from the ne­ces­sity of weigh­ing and bal­anc­ing friend groups against each other, there’s also a pre­dic­tive as­pect. If I ex­pect a given moral meme to be­come dom­i­nant over the next decade, it seems pru­dent to be “on the right side of his­tory” re­gard­less of the pre­sent im­pact on my so­cial cir­cle.

Be­ing forced to choose be­tween two so­cial groups with in­com­pat­i­ble moral stances is, un­sur­pris­ingly, stress­ful. So­cial aliena­tion is a painful pro­cess, as can at­test any Amish per­son who has been shunned. How­ever what may be worse than any clean break is the mo­ment just be­fore, try­ing to walk the knife edge of barely-over­lap­ping morals in the des­per­ate hope that the cen­tre can hold.

IV. (PostScript)

I wrote this fo­cused mostly on my­self. Hav­ing finished, I can­not help but won­der how much an ap­prox­i­ma­tion of Pos­tel’s Prin­ci­ple guides the moral prin­ci­ples of most peo­ple, whether they would ac­knowl­edge it or not. Even peo­ple who claim to de­rive their moral­ity from first prin­ci­ples of­ten end up with some­thing sur­pris­ingly close to their lo­cal so­cial con­sen­sus.