Let’s assume for the moment that the uni­verse is com­pletely, totally, 100% deterministic.

If this is the case, then given a com­plete under­stand­ing of the laws of nature and a total snap­shot of the uni­verse at some instant, a very ded­i­cated indi­vid­ual (or machine, more likely) would be able to roll out the cal­cu­la­tions and pre­dict future states of the uni­verse. In prin­ci­pal, anyway. I admit the details would prob­a­bly get a little gnarly.

So let’s say modern sci­ence has fig­ured out more or less how to do this: a means of pre­dict­ing the future with 100% cer­tainty. It’s a neat trick. Some­thing to impress people at par­ties, anyway.

Now imag­ine a sci­en­tist—the stereo­typ­i­cal labcoat-​clad clipboard-​wielding boffin will do nicely. In fact, let’s pre­tend it’s Jen, circa strip 1. She has access to the skyscraper-​sized super­com­puter that Sci­ence has con­structed for the pur­pose of brute-​forcing its way through the mind-​boggling infini­tude of cal­cu­la­tions nec­es­sary to pre­dict the future. It’s pretty cool. She’s pretty happy about it.

One day the gang is at Razor Burger for lunch and they get to talk­ing. Jen recently bought a new cat, and she’s rap­tur­ously describ­ing it to Roy. In pass­ing, she men­tions that the uni­verse is deter­min­is­tic and that sci­ence has devel­oped a method with which it can per­fectly pre­dict the future. But god is it a beau­ti­ful cat. Her name is Agnes and she’s a calico Japan­ese Bob­tail. You really need to come over and see it one day.

In the mean­time, Roy doesn’t really care about cats. I mean, they’re okay, but in his opin­ion Jen seems a smidge over­en­thu­si­as­tic. Then again, at least she’s not talk­ing about her tuba again. So it could be worse. But wait, what was this whole thing about the uni­verse being deter­min­is­tic and sci­ence pre­dict­ing the future? That sounds bad for Roy’s free will. Roy likes his free will. Quite a bit, in fact. If he has any say in the matter, he’d rather not part with it.

Roy says as much, and Jen replies, “Oh, free will? No, that’s a myth. A farce! Here, let me show you.” She whips out a hand­held device from her coat pocket. It looks like a minia­ture Etch-A-Sketch with a retractable key­board. “Have you decided what you’re ordering?”

“What? No. I’m trying to decide between the Shank-​wich and the Shuriken Salad. I’ve been trying to watch my figure, but the Shank-​wich is unde­ni­ably delectable…”

“Perfect! See, this unit is a portable inter­face to The Com­puter. I’ll just ask it to pre­dict what you even­tu­ally decide to order. It’ll spit out an answer in a minute or two; you won’t even have to bother deliberating!” Think­ing she’s doing him a small favor, Jen begins tap­ping away at the device’s key­board. She’s hits SUBMIT before Roy can retaliate:

“No? What? That’s pre­pos­ter­ous! I refuse to listen to your device. In fact, what­ever it tells me to do, I shall do the exact oppo­site! Let’s see how this Compute-O-Whiz of yours likes that!”

And now we’ve come to the real crux of the story. If the machine can pre­dict exactly how the future will unravel, then by def­i­n­i­tion what­ever answer it yields must be the thing that Roy orders. But Roy is pred­i­cat­ing his whole deci­sion on the basis of doing the exact oppo­site of what the machine tells him to do. The machine cannot be incor­rect, but Roy is infi­nitely stub­born. He would sooner be stricken dead than follow the whims of a capri­cious mechan­i­cal deity. The unstop­pable object has run into its immov­able wall. In short: a para­dox. There­fore, it is log­i­cally impos­si­ble for Jen to tell Roy what he will be ordering.

Well, okay, fine. Just don’t tell Roy. Easy enough. Unfor­tu­nately, this doesn’t really fix any­thing. See, Milo’s been remark­ably quiet this whole con­ver­sa­tion. Actu­ally, he’s been dis­tracted. Day­dream­ing about the sci­en­tific con­se­quences regard­ing the puta­tive exis­tence of lep­rechauns, mostly. But he catches the tail end of Jen and Roy’s exchange, and he finds Roy’s stub­born­ness amus­ing. He decides to try it out with his own portable inter­face to The Com­puter. If it tells Milo that he’s going to order the Cut­lass Supreme Combo, he’ll get the Masamune Masher, and vice versa.

Now that the pre­dic­tor and the stub­born person are one and the same indi­vid­ual, The Com­puter isn’t even allowed to gen­er­ate a result. Think about it: how can The Com­puter cal­cu­late a cor­rect answer if that answer is nec­es­sar­ily the one that will not come to pass? Back when it was Jen and Roy, Jen could fea­si­bly make the query, secretly record the result some­where, wait for Roy to place his order, and then reveal the (cor­rect) pre­dic­tion after the fact. There’s noth­ing log­i­cally incon­sis­tent about that. In Milo’s case, how­ever, knowl­edge of the pre­dic­tion directly results in a paradox.

The weak con­clu­sion of our little mental exer­cise is that it’s impos­si­ble for a stub­born indi­vid­ual to know their future. A stronger con­clu­sion might be that it is gen­er­ally impos­si­ble for a person to have 100% accu­rate infor­ma­tion about the future. I can envi­sion an inter­est­ing uni­verse where the weak con­clu­sion holds but the strong one does not: in this case, The Com­puter implic­itly knows who is stub­born (and at what times, if we make the rea­son­able assump­tion that folks can be tran­siently stub­born) and fudges its responses accord­ingly. That is to say, The Com­puter occa­sion­ally lies. In point of fact, The Com­puter really does “know” the accu­rate answer—but it also “knows” that reveal­ing this answer will self-​defeat the prophecy. (Of course, we should be care­ful about per­son­i­fy­ing The Com­puter with words like know: although it is in a sense “omniscient”, it also nec­es­sar­ily lacks a cer­tain degree of agency. It should, for exam­ple, be impos­si­ble for The Com­puter itself to be stubborn!)

The point is: in this the­o­ret­i­cal world, stub­born indi­vid­u­als run the risk of get­ting a false pre­dic­tion. Thus, in order to ensure that they’re always get­ting a true and cor­rect pre­dic­tion, they need to promise to them­selves that they’ll follow what­ever advice The Com­puter gives them. Talk about a self-​fulfilling prophecy!

I’ll leave you with one last inter­est­ing corol­lary of the weak con­clu­sion. It’s suc­cinctly expressed as fol­lows: If God exists and is omni­scient, then it fol­lows that God can’t be stubborn!

¹ This post is largely based on an argu­ment pre­sented in Ray­mond Smullyan’s excel­lent book This Book Needs No Title.