Welcome back to The Philosopher’s Lexicon. My primary goal in this series is to explore common philosophical vocabulary, hopefully transforming these words from useless jargon into meaningful terms. My secondary goal is to highlight how contentious some of these terms can be – especially those which seem obvious. These definitions will not be comprehensive by any means, so please feel free to add your own understanding of each term as we go.
This week’s entry into the lexicon will be the last of a string of distinctions. I began some weeks ago by discussing the de dicto/de re distinction, then moved onto the distinction between ontology and epistemology, after which I tackled logical and causal possibly, and most recently I covered the difference between analytic and synthetic reasoning. To these, I add the distinction between a priori and a posteriori knowledge, a classification which I first encountered as a philosophy student in Kant’s Groundwork for the Metaphysics of Morals and A. C. Grayling’s An Introduction to Philosophical Logic, and is explained well in Nils Ch. Rauhut’s Ultimate Questions.
In its weaker sense, a priori knowledge is knowledge that requires no external experience to comprehend. All you have to do is think things through according to the knowledge you already have. A priori knowledge can be verified without leaving your desk according to what you already know. This can refer to analytic deduction: no external validation is required to affirm the truth of tautological definitions like “A bachelor is an unmarried man” or equations like “10 + 10 = 20”. This is a weak definition of a priori knowledge because it refers only to knowledge that you learned earlier, meaning you could have learned that knowledge from earlier experience.
The stronger sense of the word refers to knowledge that is innately held. This is obviously more controversial, as it assumes that we are born with knowledge prior to our experience with the world, usually eternal truths and immutable forms. Plato’s theory of knowledge is that when we learn anything in the world, what we’re really doing is recalling truths we already knew by virtue of have a knowing soul (nous). When we think we’re leaning from experience, we’re really just reminding ourselves of things we already knew, but haven’t yet named. While some versions of Platonism claim that this means we are born with perfect geometric forms in our minds, I tend to think Plato’s intention was that we were born with more abstract concepts of order and pattern. But that’s a post for another day.
In contrast, a posteriori knowledge comes to us through our experience – literally, post or after our perceptions. A posteriori knowledge requires that we gather new information to verify, such as synthetic statements like “Bill is a bachelor” or “Bill has 10 apples in a basket, and 10 on the counter, making 20 in total”. We’ll have to go and consult with Bill to know if either of these statements are true – no amount of thinking can override our need to gather new information.
While in philosophy the terms are mostly used in reference to logic and Kant, the difference between a priori and a posteriori knowledge is essential in machine learning and artificial intelligence. Siri, Google Now, and other new technologies rely on creating systems that mix pre-programmed structures (a priori knowledge) with millions of examples gathered from the real world (a posteriori knowledge).
These terms may seem redundant when placed in context with “analytic and synthetic reasoning” and “logical and causal possibility,” but there are a few different ways frame these distinctions in context with each other. Next week, I will attempt to put all of the distinctions I’ve covered into context with each other in order to explore the various ways these terms can overlap and conflict.
Stories & Soliloquies 
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