Inferring and Explaining

11 One way of reconstructing the skeptic’s rea- soning is as follows: 1. I can be confdent that so-and-so because of such-and-such. I can be confdent that I have plenty of gas because the gauge reads three-quarters of a tank. 2. If this-and-that were true, however, such- and-such would no longer justify my conf- dence in so-and-so. If the gauge were broken, however, its reading three-quarters of a tank wouldn’t justify my confidence that I have plenty of gas. 3. Tis-and-that is possible, and I cannot prove that this-and-that is not true. Te gas gauge could be malfunctioning, and I have no proof that it is not. 4. Terefore, since such-and-such cannot be ruled out, I can no longer be confdent that so-and-so. Terefore, since the gauge’s being broken cannot be ruled out, I can’t be sure that I have plenty of gas. Tere are many circumstances where this kind of skeptical thinking is demanded. A sales- man calls you on the phone and ofers to sell you stock worth more than one thousand dollars a share for only a hundred. You damn well bet- ter go through something such as the following reasoning: 1. I can be confdent that the stock is a good deal because the salesman told me so. 2. If he’s lying, however, I can’t trust him. 3. It’s possible that he’s lying, and I cannot prove that he’s not lying. 4. Terefore, since his lying cannot be ruled out, I can no longer be confdent that the stock is worth so much. One of the oldest questions in philosophy is whether this kind of skeptical reasoning can be generalized across the board. Should our intellectual confidence in so-and-so—what our senses tell us, the word of scientists, or whatever—be undermined by our failure to rule out some confdence-undermining possibility? Dreaming and the External World Here’s a general purpose skeptical argument—a confdence-undermining possibility—that may well have already occurred to you.What if you’re not really reading this but just having a vivid dreamabout reading it? Doesn’t the possibility of life’s being a dream or any particular instant of it being a dream rule out the possibility of any kind of knowledge? Some philosophers have suggested that it might. One whole school of philosophy claims that the sensesmust be the ultimate source of all sub- stantive knowledge. Other schools do not insist that the sensesmust produceall knowledge; there may be other sources as well. All parties agree, however, that the senses are directly involved in most of what we claim to know. Tat’s what makes the dream hypothesis so serious. In one fell swoop, it shakes our confdence in everything the senses have to say. You believe that you’re in trouble in your philosophy class because of the poor grade at the top of your termpaper. But if it skePtICIsm