section_path: Sources
section_name: Sources
paragraph_text: Sources
  
  Sources of knowledge are ways in which people come to know things. They can be understood as cognitive capacities that are exercised when a person acquires new knowledge. Various sources of knowledge are discussed in the academic literature, often in terms of the mental faculties responsible. They include perception, introspection, memory, inference, and testimony. However, not everyone agrees that all of them actually lead to knowledge. Usually, perception or observation, i.e. using one of the senses, is identified as the most important source of empirical knowledge. Knowing that a baby is sleeping is observational knowledge if it was caused by a perception of the snoring baby. However, this would not be the case if one learned about this fact through a telephone conversation with one's spouse. Perception comes in different modalities, including vision, sound, touch, smell, and taste, which correspond to different physical stimuli. It is an active process in which sensory signals are selected, organized, and interpreted to form a representation of the environment. This leads in some cases to illusions that misrepresent certain aspects of reality, like the Müller-Lyer illusion and the Ponzo illusion.
  
  Introspection is often seen in analogy to perception as a source of knowledge, not of external physical objects, but of internal mental states. A traditionally common view is that introspection has a special epistemic status by being infallible. According to this position, it is not possible to be mistaken about introspective facts, like whether one is in pain, because there is no difference between appearance and reality. However, this claim has been contested in the contemporary discourse and critics argue that it may be possible, for example, to mistake an unpleasant itch for a pain or to confuse the experience of a slight ellipse for the experience of a circle. Perceptual and introspective knowledge often act as a form of fundamental or basic knowledge. According to some empiricists, they are the only sources of basic knowledge and provide the foundation for all other knowledge.
  
  Memory differs from perception and introspection in that it is not as independent or basic as they are since it depends on other previous experiences. The faculty of memory retains knowledge acquired in the past and makes it accessible in the present, as when remembering a past event or a friend's phone number. It is generally seen as a reliable source of knowledge. However, it can be deceptive at times nonetheless, either because the original experience was unreliable or because the memory degraded and does not accurately represent the original experience anymore.
  
  Knowledge based on perception, introspection, and memory may give rise to inferential knowledge, which comes about when reasoning is applied to draw inferences from other known facts. For example, the perceptual knowledge of a Czech stamp on a postcard may give rise to the inferential knowledge that one's friend is visiting the Czech Republic. This type of knowledge depends on other sources of knowledge responsible for the premises. Some rationalists argue for rational intuition as a further source of knowledge that does not rely on observation and introspection. They hold for example that some beliefs, like the mathematical belief that 2 + 2 = 4, are justified through pure reason alone.
  
  Testimony is often included as an additional source of knowledge that, unlike the other sources, is not tied to one specific cognitive faculty. Instead, it is based on the idea that one person can come to know a fact because another person talks about this fact. Testimony can happen in numerous ways, like regular speech, a letter, a newspaper, or a blog. The problem of testimony consists in clarifying why and under what circumstances testimony can lead to knowledge. A common response is that it depends on the reliability of the person pronouncing the testimony: only testimony from reliable sources can lead to knowledge.