Trying To Think
Thursday, March 11, 2004
Lecture 2 (10 March 2004)

Learnt about a few distinctions:

1. a priori vs a posteriori. An epistemic distinction (ie. a distinction about how we know). A priori knowledge is known without experience, a posteriori can only be known through experience. eg. "2+2=4" vs "rain is wet".

2. analytic vs synthetic. A semantic distinction (ie. to do with meaning). Analytic statements have the truth or falsity contained within the statement, synthetic statements need reference to other facts. eg. "all bachelors are unmarried" vs "John is a bachelor".

3. necessity vs contingency. Necessary truths (or falsehoods) have to be true or false - it's not possible for them to be otherwise. Contingent truths may have been true or false; they happen to be either true or false, but it would have been possible for them to be otherwise. eg. "2+2=4" vs "dinosaurs are extinct"

Within necessity and contingency we have the logical (or "conceptual") and nomological (or "physical") distinction. If logical, the necessity or contingency covers all possible worlds, if nomological, it only covers worlds with the same laws of nature as ours. If something is nomologically necessary, it is true in all worlds with laws of nature like ours, if it is nomologically contingent, then it is possible in all worlds with laws of nature like ours (so there will be at least one with it true, and at least one with it false). "dinosaurs are extinct" is nomologically contingent, "water is wet" is nomologically necessary, but not logically necessary (it is possible to conceive of different laws of nature where H2O is not wet, although arguably it would no longer be "water" - more on this later).

Logical is the default and usual meaning of necessity and contingency.

Coextensiveness Thesis

a priori = analytic = logically necessary (and therefore nomologically necessary as well)
a posteriori = synthetic = logically contingent (nomologically ??? - work this out)

Arguments against Coextensiveness Thesis

Kant: some statements are a priori and synthetic eg. causation and maths. "every event has a cause" is a priori, but not true simply by virtue of the words.
Saul Kirpke: there are necessary truths that are a posteriori (eg. "Water is H20" - must be the case, but known by experience) and contingent truths that are a priori (eg. "The metre bar in Paris is one metre long" - depends on no other facts, but could have been otherwise).

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