text: [Themes: ghost, surreal, mathematics]
  
  > look around
  Gray Matter, Level Four
  This space is made of the same gray matter as the three above you. There is an inscription on the wall, as well as exits up to Gray Matter, Level Three, north to the Smoky Pub, south to the Window into Oblivion, east to the Machine Room, and west to the Research Lab.
  
  > Go south
  You make your way south. As you do, you feel an electrical impulse course through you. You find yourself in...
  
  A tray is attached to the wall next to the window.
  
  Through the window you can see a flat, gray plane. A pile of discarded objects lies a short distance away in the plane. Standing near the pile, rocking back and forth and muttering to himself, is René Descartes.
  
  > check what you're carrying
  You are carrying:
  Disquisitiones Arithmeticae
  the largest integer
  a logic scroll
  some limit shoes (being worn)
  a square root
  a set (no longer empty)
  Descartes's saying
  a printout
  a toy mountain range
  some correspondence
  the trig homework
  your math book
  
  > put largest in the tray
  You place the largest integer on the tray. The tray beeps loudly, and a voice says, "Non-existence detected!"
  
  A door slides open in the wall, and a woman runs in. She's holding a cell phone and is slightly out of breath. The door closes behind her, leaving only a faint outline.
  
  "I'm so sorry," she says. "Between my cell phone constantly ringing and all the cranks on the Internet, I just can't keep up like I used to."
  
  Turning to the tray, she picks up the largest integer. "Hmmm." she says. "This definitely does not exist. How did you acquire it? This was consigned to Oblivion at least by Archimedes's time. Actually, never mind. I bet one of the imps left it somewhere; they're not always reliable. It's enough that we have it back."
  
  She makes a quick call on her cell phone. An imp materializes, takes the largest integer from the woman, rematerializes outside the window, and drops the largest integer onto the pile. Then the imp disappears.
  
  "Where are my manners?" the woman asks. "I'm Hypatia, caretaker of Oblivion."
  
  > greet Hypatia
  "Thank you for returning the largest integer," Hypatia says. "As you likely have already figured out, Oblivion is where we keep everything that doesn't exist. I'm the caretaker. This job was a sort of reward for being one of the few mathematicians martyred.
  
  "If you know of anything else that doesn't exist, please let me know, and I'll take care of it," Hypatia says. "I also field questions about whether things exist." She leans closer and whispers, "Occasionally we even make a mistake and consign something to Oblivion that actually does exist. We don't really like to talk much about that, though."
  
  Hypatia's cell phone rings, and she answers it. "No," Hypatia says. "I don't think there is an odd perfect number, either, but that proof isn't valid. Please try again." She hangs up.
  
  > ask about descartes
  (to Hypatia)
  You point through the window at Descartes, who is still rocking back and forth and muttering to himself.
  
  Hypatia's eyebrows rise. "What is he doing there? That's a mistake; surely Descartes actually exists!" She sighs. "I've been too preoccupied."
  
  She thinks for a moment and then comes to a decision. "I bet one of the imps did this, but I don't know which one, and I don't have time right now to sort this out. If you can figure out why someone thought Descartes does not exist I'll do what I can to get him out of Oblivion."
  
  Hypatia's cell phone rings once more, and she answers it. Hypatia listens for a while. Finally, exasperated, she cuts the caller off. "No, no, no! That doesn't work! Besides, do you really think you're going to be the one to prove that there is no nontrivial solution to Riemann's zeta function with real part unequal to 1/2? Far better minds than yours have worked on that and failed!" She ends the call with an emphatic jab of her finger.
  
  > show saying to Hypatia
  (first taking Descartes's saying)
  Hypatia takes the saying from you. "This means 'I think; therefore I am.'" She thinks for a moment. "Since the saying is about existence it might have something to do with one of my imps consigning Descartes to Oblivion. But Descartes is deducing that he exists, not that he doesn't exist. There must be more to it than this." She returns the saying.
  
  Hypatia's cell phone rings again, and she answers it. "I'm sorry, but I'm afraid I don't know how to say 'exist' in Old Gywndyngwah," Hypatia says. "I'm not familiar with that language." She ends the call.
  
  > greet descartes
  (René Descartes)
  Descartes looks up at you briefly, a request for help in his eyes. Then he grabs his head once more. "I can't think here. I can't think here," he mutters.
  
  Hypatia's cell phone rings again, and she answers it. "God?!"
  Hypatia flashes a grin at you. "Lots of people have tried, but as yet there is no conclusive proof that God does not exist." She hangs up and turns to you conspiratorially. "Don't tell anyone, but who do you think gave me this job?"
  
  > Inventory
  You are carrying:
  Descartes's saying
  Disquisitiones Arithmeticae
  a logic scroll
  some limit shoes (being worn)
  a square root
  a set (no longer empty)
  a printout
  a toy mountain range
  some correspondence
  the trig homework
  your math book
  
  Hypatia's cell phone rings again, and she answers it. "No," Hypatia says. "You're likely right, but there's no proof yet that there is no polynomial-time algorithm for finding Hamiltonian cycles." She hangs up.
  
  > show scroll to Hypatia
  Hypatia replies, "Yes, these are two well-known logical fallacies. What do they have to do with Descartes being consigned to Oblivion?"
  
  Hypatia's cell phone rings once more, and she answers it. Hypatia listens for a while. "No," she says. "There are some intelligent and thoughtful people in the other political party." She holds the phone away from her ear, and you hear angry ranting on the other end. Hypatia lets it go on for a while before she breaks in with "You might try spending less time on social media," before hanging up. She turns to you. "It seems like that has been getting worse lately."
  
  > show saying to Hypatia
  "Descartes's saying 'I think; therefore I am,' might have something to do with one of my imps consigning Descartes to Oblivion. But Descartes is deducing that he exists, not that he doesn't exist.
  There must be more to it than this."
  
  Hypatia's cell phone rings again, and she answers it. "Just because we haven't found life on other planets yet doesn't mean it doesn't exist," Hypatia says. "Absence of evidence isn't evidence of absence." She ends the call.
  
  > Show descartes to Hypatia
  (Descartes's saying to Hypatia)
  "Descartes's saying 'I think; therefore I am,' might have something to do with one of my imps consigning Descartes to Oblivion. But Descartes is deducing that he exists, not that he doesn't exist.
  There must be more to it than this."
  
  Hypatia's cell phone rings once more, and she answers it. "No," Hypatia says. "I sympathize, but there are some honest lawyers." She hangs up.
  
  > show correspondence to Hypatia
  (first taking the correspondence)
  Hypatia takes the correspondence from you and reads through it. "This is interesting! It appears the last thing Descartes did was write this letter to Fermat. Perhaps it has something to do with why one of my imps consigned Descartes to Oblivion. But why?" She returns the correspondence.
  
  "Wait a minute..." Hypatia stops herself. "'I think; therefore I am.' That's Descartes's famous saying. What was that last phrase in Descartes's correspondence? 'I think not...' Oh, no!" She buries her face in her hands.
  
  When Hypatia raises her head again she has an exasperated look on her face. "I think we both know what happened here," she says. "I could order my imps to set Descartes free now, but there's no guarantee that they won't make this kind of mistake again.
  
  "I hate to ask you for something else, but I really need some evidence to show my imps that 'I think; therefore I am,' and 'I think not,' do not together imply 'I am not.' If you can bring some evidence like that to me, I'll feel comfortable letting Descartes go."
  
  Hypatia's cell phone rings once more, and she answers it. Hypatia listens for only a moment before she says, "Wait a minute. You state that truth does not exist. Yet that claim itself is a truth claim. You're assuming the existence of the very concept you claim doesn't exist!" She hangs up emphatically and says to you, "I'll be so glad when the postmodernism fad passes."
  
  > show scroll to Hypatia
  Hypatia takes the logic scroll from you. "Yes, these are two well-known logical fallacies. And denying the antecedent is exactly the mistake that my imps made. Well done!"
  
  Hypatia makes a call on her cell phone, and an imp materializes. "One of you idiots consigned Descartes to Oblivion just because he said, 'I think not'!" Hypatia yells. "Don't you fools even know basic logic?!"
  
  She hands the imp the logic scroll and begins tapping her foot. The imp reads through the scroll. He blanches.
  
  "Get Descartes out of there, now!" Hypatia orders.
  
  The imp disappears and then rematerializes on the other side of the window. He grabs Descartes by the arm, and they both vanish.
  
  Hypatia turns back to you. "Thank you for straightening that out," she says. "My imp will be returning Descartes to wherever he was before. Hopefully we won't be seeing that kind of mistake again.
  
  "If you don't mind, though, I probably should get back to my computer." She sighs. "The Internet has brought out all of the cranks from whatever holes they normally hide in.
  
  "Speaking of which..." She hands you a small object. "People keep giving me these as gag gifts. You might it useful. Also, if you mess it up, bring it back, and I'll give you a new one. Just knock on the door to get my attention."
  
  Hypatia presses a button on her phone, and the door in the west wall slides open. She walks through it. As the door closes behind her, she calls out, "You might want to check on Descartes; he will probably be grateful for what you've done!"
  
  [Your understanding of Logic has increased. You estimate that
  your score on tomorrow's final has just gone up by four
  points.]
  
  > check your inventory
  You are carrying:
  a small crank
  some correspondence
  Descartes's saying
  Disquisitiones Arithmeticae
  some limit shoes (being worn)
  a square root
  a set (no longer empty)
  a printout
  a toy mountain range
  the trig homework
  your math book
  
  > examine the crank
  This crank is made of lead. It has threads that are oriented clockwise.
  
  > go to the north
  As you move north, you feel an electrical impulse course through you. The space around you seems to fold in on itself.
  
  Gray Matter, Level Four
  
  > Go upwards
  You make your way up.
  
  Gray Matter, Level Three
  
  > go south
  You make your way south. As you do, you feel an electrical impulse course through you. You find yourself in...
  
  > go east
  Descartes has left you a copy of his La Géométrie here.
  
  A display is set into the wall, with a slot next to it.
  
  You can also see a thin card here.
  
  > Go east
  From here, the only exit is west.