It has long been pointed out by opponents of skepticism that such an attitude cannot be taken to its extreme conclusion in the real world in which we operate- even skeptics must live their lives according to rules they must rely upon not to fail. Skepticism (and all philosophy) cannot avoid the cumbersome nature of human language and the simple fact that the only tool humans have to investigate the nature of existence is the brain, which spawns coherent thoughts in a lingual form. In day to day life, realistically, the observations and calculations of the brain must be relied upon to reach trustworthy conclusions. The ardent skeptic will certainly not step off of a cliff unless he has a death wish. Obviously anyone who disregarded the law of gravity would be called insane. Therefore, it seems reasonable to accept the reliability of the laws of science. When arguing for his skeptical position, the skeptic would be forced to call into question the reliability of these laws in his refutation of absolute truth, attainable knowledge, etc. This is an apparently irrational behavior by the skeptic- to live by laws that he will later claim unknowable or altogether false. If one chooses to ignore this obvious fact, there is little that can be done to argue, for argument assumes that rational thought processes are at work. As Hank Hanegraaff has said, "Even those who deny reality look both ways before they cross the street."
One popular example skeptics use to disprove attainable knowledge is the Brain in the Vat argument. "The Brain in the Vat scenario is a thought experiment designed to show the plausibility of radical skepticism. If we were merely Brains in Vats, hooked to computers which send the same sorts of electrical impulses that a normal body would have, we would have experiences indistinguishable from "real life" experiences with no hope of discovering our predicament. Thus, it is possible, according to those who find this example compelling, that radical skepticism, the thesis that we have no certain knowledge, is true. The Brain in a Vat scenario was responded to by Hilary Putnam to promote his version of the argument for the claim that radical skepticism has to be false. He uses a version of the private language argument to argue that a brain in a vat would have no true beliefs. A brain in a vat which was causally hooked up to receive the same inputs it would have received if it had been in a body would, ex hypothesi the radical skeptic's theory, believe the same things we believe. But it would have no public referent for its words and sentences, and hence couldn't have any human language, since on Putnam's view, all languages require public referents. Thus, he argues that the skeptic who thinks all of our belief are (or could be) false has to suppose that we are like brains in vats. But since brains in vats could not have a language, and we do..."
Rudolf Carnap and Skepticism
"Can we know that things exist independent of our linguistic frameworks? This, of course, is the traditional problem of skepticism. Carnap has a relatively straightforward answer to the skeptic: ``To be real in the scientific sense means to be an element of the system; hence this concept cannot be meaningfully applied to the system itself.'' In other words, it is meaningless to make claims about objects independent of linguistic frameworks. Carnap would see this as analogous to asking questions about the absolute ontological status of `first base' independently of the game of baseball. Accusing a question of being ``meaningless'' is not Carnap's last word on external questions. Carnap allows that some kinds of pragmatic criteria can be used in choosing a linguistic framework. In fact, he...