A Priori vs. A Posteriori If I know something, I must have justification. If justification essentially relies on sensory experience, then it is a posteriori justification and my knowledge is a posteriori knowledge. Examples of such knowledge are: It is raining (I see it is). It was snowing yesterday (I remember seeing it.) It is raining in New York (my brother can see it and tells me it is.) In the second case, the sensory information is recalled by a reliable memory; in the third, transmitted by a reliable witness. If I know something essentially through reason, then my justification is a priori justification and my knowledge is a priori. Examples of such knowledge are: 2+2=4 The area of a triangle is ½ x base x height.

What do the terms mean? A posteriori means afterwards – think of posterior – and a priori means before – think of prior. Intuitively - a priori knowledge: ‘before’ applying my senses. I can prove something is true all ‘in my mind’. When I have a posteriori knowledge, I need to apply my senses. (N.B. I need to use my powers of reason as well. If I see it is raining, then my senses deliver information to my mind but it is reason that concludes it is raining through applying the concept RAIN.) A crude way of thinking about the distinction. A priori knowledge is ‘armchair knowledge’. A posteriori knowledge is ‘in the field knowledge’. You have to get up and look or listen or smell or taste or feel – you need to gather evidence.

Necessary and Contingent This is a distinction between facts or states of affairs. Something is necessary if it could not have been otherwise. It is a necessary truth that bachelors are unmarried and that triangle have three sides. There is no scenario or possible world in which things could have been otherwise. Something is contingent if it could have been otherwise. It is actually a matter of fact that war broke out in 1914 and that Paul Sheehy exists. However, the world could have been such that there was no WW1 and no Paul Sheehy. To that something is a necessary truth is to say that it is true in all possible worlds. Something is contingently true if true in at least one possible world.

Epistemology (knowledge- related) Metaphysics (existence –related) NecessaryContingent A priori Knowing through reason 2+2=4 There is an infinitude of primes. God exists. Mind and body are distinct substances. A posteriori Knowing through experience The Earth revolves around the sun. Paris is the capital of France. Descartes died in 1650.

Rationalism maintains that knowledge of reality is a priori. Rationalism claims that reason allows us to know necessary truths ranging from mathematics to ethics to metaphysics. The necessary truths are not just tautologies, but tell us about the nature of reality: what exists and what it is really like. Reason provides the foundation of general principles or truths on which detailed scientific enquiry can be based.