1.1.7.1.1 Reasoning Techniques: Types of Propositions

Version 1.3 March 2013                                (Previous Version)

There is lots of more information on these technical philosophical distinctions.  Most of us don't need to bother with this level of detail, so skip or skim over this page if it looks boring.

Philosophers classify certain and uncertain knowledge in a few different overlapping ways:

What we know for certain is called a priori knowledge (rather than a posteriori knowledge). 

●   A priori knowledge or justification is independent of experience: we don't have to examine how things are in the physical world, as we do in science. 

      For example: “All bachelors are unmarried�.  That is what being a bachelor means.

●   A posteriori knowledge or justification is dependent on experience or empirical evidence.

For example: “Some bachelors are very unhappy�.  We would need to survey bachelors to know this.  We cannot work it out from the meaning of the words.

Certain knowledge is stated in analytic propositions rather than in synthetic propositions.

●   Analytic propositions are true by virtue of their meaning, without regard to any facts.

For example: “This triangle has 3 sides�.  That’s what “triangle� means.

●   Synthetic propositions are true by how their meaning relates to the world.

For example: “This polygon has 13 sides�.  We would need to count the number of sides.  We cannot work it out from the meaning of the word polygon.

Certain knowledge is stated in propositions that are necessary rather than contingent:

●   A necessary proposition is always true or always false, by definition.

●   The truth of a contingent proposition is contingent upon the truth of the sentences which comprise it.  Contingent propositions depend on the facts.

Certain knowledge is in contradictions or tautological propositions rather than possible ones.

●    Tautological propositions must be true, no matter what the facts are or could be.

For example: “The sky is blue or the sky is not blue.�.

●    Contradictions must necessarily be false, no matter what the facts are or could be.

For example: “It’s raining and it's not raining.�  It cannot be both. 

●    Possible propositions, are true or could have been true given certain circumstances.

For example: “There are only three planets.�  Not true, but it might have been once.

What we know for certain is, in essence, true by definition, regardless of the facts.