section_path: Central concepts/Justification/Sources
section_name: Sources
paragraph_text: Sources
  
  Sources of justification are ways or cognitive capacities through which people acquire justification. Often-discussed sources include perception, introspection, memory, reason, and testimony, but there is no universal agreement to what extent they all provide valid justification. Perception relies on sensory organs to gain empirical information. There are various forms of perception corresponding to different physical stimuli, such as visual, auditory, haptic, olfactory, and gustatory perception. Perception is not merely the reception of sense impressions but an active process that selects, organizes, and interprets sensory signals. Introspection is a closely related process focused not on external physical objects but on internal mental states. For example, seeing a bus at a bus station belongs to perception while feeling tired belongs to introspection.
  
  Rationalists understand reason as a source of justification for non-empirical facts. It is often used to explain how people can know about mathematical, logical, and conceptual truths. Reason is also responsible for inferential knowledge, in which one or several beliefs are used as premises to support another belief. Memory depends on information provided by other sources, which it retains and recalls, like remembering a phone number perceived earlier. Justification by testimony relies on information one person communicates to another person. This can happen by talking to each other but can also occur in other forms, like a letter, a newspaper, and a blog.