A PRIORI AND A POSTERIORI KNOWLEDGE Introduction to Philosophy Jeff Strayer Two important terms of epistemology are ‘a priori’ and ‘a posteriori.’ Philosophy concerns both things which can only be known through experience or a posteriori, such as that fire burns and that works of art are pleasurable to look at, and things which can be approached through reflection or a priori, such as the laws of logic and mathematics. a posteriori - [L. literally, from the latter, from what comes after] concerns knowledge gained through experience [Gould] a priori - [L. literally, from the former, from what comes before] concerns knowledge gained prior to experience [Gould] or, better, independently of experience in not depending directly on experience These terms concern how we as knowers stand in relation to objects known, and that is why they are both terms of epistemology. If we can only know that something is true or false on the basis of experience alone then that knowledge is said to be a posteriori. For example, if we want to know if it is true or false that there are four people in the room across the hall we can only determine that on the basis of experience, and not on the basis of reason. “An a posteriori proposition can be known to be true or false only by reference to how, as a matter of contingent fact, things have been, are, or will be” - Antony Flew - A Dictionary of Philosophy. (‘Proposition,’ as used here, is another term for ‘statement.’ And whatever is contingent is not logically necessary; it is an accident or might not have been. Thus the planet Earth is a logical accident since there is nothing at all logically necessary about its existence.) On the other hand, if we can know that something is true or false without having to consult experience in each case, but can understand the truth or falsity of a statement by understanding the meaning of the terms involved in the statement, then that knowledge is said to be a priori. For example, if we understand basic arithmetic we know that any two people and any other two people make four people, whether they are in the room across the hall or not. And in a more general sense we know that any two things and any other two things together make four things, and we can know this without having to go and count in each case, if we merely know what ‘two’, ‘four’ and ‘plus’ or ‘addition’ mean. Because 2+2=4 depends simply on the meaning of the terms involved and the relation between them we do not have to check in each case to see if two and two will make four. We can see, a priori, that they must and so always will. Gould’s definition of a priori in the glossary is misleading, since we cannot know anything prior to experience. For instance, in our example we must have been taught a certain amount of mathematics to understand what we are talking about. But once we understand the meaning of the terms and the rules of arithmetic we no longer need experience in each case to substantiate the truth or falsehood of such a statement. A better definition is provided by Flew here from the same work previously quoted: “An a priori proposition is one which can be known to be true or false without reference to experience, except in so far as experience is necessary for understanding its terms.” Truths of reason such as logic and mathematics are thought by most philosophers to be a priori and necessary, and truths of fact, or empirical truths, are normally said to be a posteriori and contingent. contingent - something that may be but also may not be (Gould); something that is not necessary. Empirical facts, such as that there is a building in New York called the Empire State Building, are contingent. The Empire State Building is a contingent entity since there might have been no such building. Contingent statements concern contingent matters of fact. For instance, “I have a younger brother” is a true statement concerning a contingent matter. It is contingent since I might not have had a younger brother. “I have an older brother” is a false statement concerning a contingent matter. It is false since I do not have an older brother, but it is contingent because I might have had one - there was nothing which logically prevented my parents from having a child before me. necessary - something that must be. A statement is necessary when what it asserts could not be otherwise, or when its denial is self-contradictory. That 2+2=4 is necessary since its contradictory is self-contradictory. That is, that 2+2 does not equal four contradicts the meanings of the terms of the statement, as does “some bachelors are married men.” There can be necessary relations between statements which concern contingent matters of fact. If that I am the first born in my family states a fact, then the statement that I have an older brother or sister is necessarily false. It is necessarily false since I am now talking about a logical or conceptual relation between statements that happen to pertain to a matter of fact. The meaning of ‘first born’ logically excludes or is logically incompatible with that of ‘older sibling.’ You must learn through experience, or a posteriori, that I am the first born in my family, but since you understand the meaning of ‘first born,’ how it relates to other things, and so what that meaning excludes as possible, you understand a priori, and so without having to ask me, that I have no older brother or sister. An analytic statement is one whose contradictory is self-contradictory. Thus the contradictory of “all bachelors are unmarried men” is “some bachelor is a married man.” That latter statement contradicts the meanings of both ‘bachelor’ and ‘married man,’ in that the meaning of one of these terms excludes the other. Another way to think of analytic statements is that the predicate of the statement simply repeats all or part of the subject of the statement in the same or synonymous language. Thus, “all bachelors are bachelors,” “all red objects are red,” and “all red objects are objects,” and “all cars are automobiles” are analytic statements. An analytic statement is one which we can know a priori to be true once we have had the experience of understanding the terms involved in the statement. A synthetic statement is one which is not analytic, or is one whose contradictory is not self- contradictory. “Some bachelors like movies” is synthetic, since that no bachelor likes film is not self- contradictory. “Some objects are red,” “there is a building in Ft. Wayne which is more than ten stories high,” and “there is a building in Ft. Wayne which is more than two hundred stories high” are each synthetic. “Some triangles are drawn in black lines” is synthetic, while “all triangles have three sides” is analytic. A synthetic statement is one which we can only know a posteriori to be true or false, since that knowledge can only come through experience. For instance, we can only know that the statement “no classroom at IPFW lacks a window” is true or false based upon experience. That is, our judging the statement to be true or false must be based either on our observation of all the buildings on campus, or on the reliable testimony of someone who has perceived all the classrooms on campus.