section_path: Limits
section_name: Limits
paragraph_text: Limits
  
  The problem of the limits of knowledge concerns the question of which facts are unknowable. These limits constitute a form of inevitable ignorance that can affect both what is knowable about the external world as well as what one can know about oneself and about what is good. Some limits of knowledge only apply to particular people in specific situations while others pertain to humanity at large. A fact is unknowable to a person if this person lacks access to the relevant information, like facts in the past that did not leave any significant traces. For example, it may be unknowable to people today what Caesar 's breakfast was the day he was assassinated but it was knowable to him and some contemporaries. Another factor restricting knowledge is given by the limitations of the human cognitive faculties. Some people may lack the cognitive ability to understand highly abstract mathematical truths and some facts cannot be known by any human because they are too complex for the human mind to conceive. A further limit of knowledge arises due to certain logical paradoxes. For instance, there are some ideas that will never occur to anyone. It is not possible to know them because if a person knew about such an idea then this idea would have occurred at least to them.
  
  There are many disputes about what can or cannot be known in certain fields. Religious skepticism is the view that beliefs about God or other religious doctrines do not amount to knowledge. Moral skepticism encompasses a variety of views, including the claim that moral knowledge is impossible, meaning that one cannot know what is morally good or whether a certain behavior is morally right. An influential theory about the limits of metaphysical knowledge was proposed by Immanuel Kant. For him, knowledge is restricted to the field of appearances and does not reach the things in themselves, which exist independently of humans and lie beyond the realm of appearances. Based on the observation that metaphysics aims to characterize the things in themselves, he concludes that no metaphysical knowledge is possible, like knowing whether the world has a beginning or is infinite.
  
  There are also limits to knowledge in the empirical sciences, such as the uncertainty principle, which states that it is impossible to know the exact magnitudes of certain certain pairs of physical properties, like the position and momentum of a particle, at the same time. Other examples are physical systems studied by chaos theory, for which it is not practically possible to predict how they will behave since they are so sensitive to initial conditions that even the slightest of variations may produce a completely different behavior. This phenomenon is known as the butterfly effect.
  
  The strongest position about the limits of knowledge is radical or global skepticism, which holds that humans lack any form of knowledge or that knowledge is impossible. For example, the dream argument states that perceptual experience is not a source of knowledge since dreaming provides unreliable information and a person could be dreaming without knowing it. Because of this inability to discriminate between dream and perception, it is argued that there is no perceptual knowledge of the external world. This thought experiment is based on the problem of underdetermination, which arises when the available evidence is not sufficient to make a rational decision between competing theories. In such cases, a person is not justified in believing one theory rather than the other. If this is always the case then global skepticism follows. Another skeptical argument assumes that knowledge requires absolute certainty and aims to show that all human cognition is fallible since it fails to meet this standard.
  
  An influential argument against radical skepticism states that radical skepticism is self-contradictory since denying the existence of knowledge is itself a knowledge-claim. Other arguments rely on common sense or deny that infallibility is required for knowledge. Very few philosophers have explicitly defended radical skepticism but this position has been influential nonetheless, usually in a negative sense: many see it as a serious challenge to any epistemological theory and often try to show how their preferred theory overcomes it. Another form of philosophical skepticism advocates the suspension of judgment as a form of attaining tranquility while remaining humble and open-minded.
  
  A less radical limit of knowledge is identified by falliblists, who argue that the possibility of error can never be fully excluded. This means that even the best-researched scientific theories and the most fundamental commonsense views could still be subject to error. Further research may reduce the possibility of being wrong, but it can never fully exclude it. Some fallibilists reach the skeptical conclusion from this observation that there is no knowledge but the more common view is that knowledge exists but is fallible. Pragmatists argue that one consequence of fallibilism is that inquiry should not aim for truth or absolute certainty but for well-supported and justified beliefs while remaining open to the possibility that one's beliefs may need to be revised later.