Eli5 A Priori vs A Posteriori.


I’m trying to get through an introduction to Kants critique of pure reason. Is there a helpful mnemonic or trick to keeping the two straight?
I know A priori is reasoning through deduction, and that a posteriori is reasoning through experience or observations. But I have to keep looking it up to keep it straight when I read it.

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The terms were introduced to me in statistics course. Here is how my professor described them:

Let’s say tossing a coin. A priori is deductive right? So based on facts, the coin has two sides and we know that. So the chances of getting heads or tails would be both 1/2.

However, a posteriori is all about experience and prior data. Let’s say you did an experiment of tossing coins. Out of 100 tosses, 70 of those landed on heads while 30 landed on tails. Based from that given data, you can say that the chances of getting heads would be 7/10 while tails are 3/10.

Hope this helps.

“A priori” is Latin for “from the earlier”. You can remember this because “prior” means coming from before in time.

“A posteriori” is Latin for “from the later”. You can remember this because often adding “post-” whatever means it is after that thing. Postmodernism, postmortem (after death, “mortem” being Latin for death), postgame, etc.

Once you link the words to the vocabulary you should already know it becomes a lot easier. If you are reasoning it out before you see it you are doing it a priori. If you are figuring it out after seeing it you are doing it a posteriori.

Basically, *a priori* things are things that must be true, even without evidence. A triangle has three sides. Something exists. They’re essentially pure logic at work, and should apply no matter what the state of the universe is if logic is correct.

*A posteriori* reasoning is figuring out something from the evidence you have available. Gold weighs 19.2 grams/cc. Dogs have four legs. Fire is hot.

The big difference is that things reasoned *a priori* must be true, if logic is right. Things derived from *a posteriori* reasoning may be true if the measurement of those things is correct and the reasoning about them is correct.

You might find a three-legged dog, or some odd isotope of gold that’s more dense than the norm, or fire that’s cool to the touch. You will never find a two-dimensional closed geometric figure with three angles but five sides.