section_path: Types/A priori and a posteriori
section_name: A priori and a posteriori
paragraph_text: A priori and a posteriori
  
  The distinction between a priori and a posteriori knowledge depends on the role of experience in the processes of formation and justification. To know something a posteriori means to know it based on experience. For example, by seeing that it rains outside or hearing that the baby is crying, one acquires a posteriori knowledge of these facts. A priori knowledge is possible without any experience to justify or support the known proposition. Mathematical knowledge, such as that 2 + 2 = 4, is traditionally taken to be a priori knowledge since no empirical investigation is necessary to confirm this fact. In this regard, a posteriori knowledge is empirical knowledge while a priori knowledge is non-empirical knowledge.
  
  The relevant experience in question is primarily identified with sensory experience. Some non-sensory experiences, like memory and introspection, are often included as well. Some conscious phenomena are excluded from the relevant experience, like rational insight. For example, conscious thought processes may be required to arrive at a priori knowledge regarding the solution of mathematical problems, like when performing mental arithmetic to multiply two numbers. The same is the case for the experience needed to learn the words through which the claim is expressed. For example, knowing that "all bachelors are unmarried" is a priori knowledge because no sensory experience is necessary to confirm this fact even though experience was needed to learn the meanings of the words "bachelor" and "unmarried".
  
  It is difficult to explain how a priori knowledge is possible and some empiricists deny it exists. It is usually seen as unproblematic that one can come to know things through experience, but it is not clear how knowledge is possible without experience. One of the earliest solutions to this problem comes from Plato, who argues that the soul already possesses the knowledge and just needs to recollect, or remember, it to access it again. A similar explanation is given by Descartes, who holds that a priori knowledge exists as innate knowledge present in the mind of each human. A further approach posits a special mental faculty responsible for this type of knowledge, often referred to as rational intuition or rational insight.