Difference between revisions of "A priori, a posteriori"

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===Most generally===
 
Basically,
 
Basically,
 
''A priori'' roughly means something "(from) that which goes before".  
 
''A priori'' roughly means something "(from) that which goes before".  
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In evidence-based science this is relatively straightforward.
 
  
It gets a little more interesting around their philosophical uses (timing and [[epistemology]]).
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===Knowledge (philosophically)===
  
And sometimes these two are mixed.
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While in general a priori can be translated as 'pre-existing',
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once you start saying 'a priori '' knowledge' '',
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you trip into epistemology (a.k.a. 'what can we know'), and a bit of metaphysics.
  
  
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'''A priori knowledge''': are things that can be knowable independently of experience/evidence {{comment|(pedantry: ...aside from the experience of the language to communicate it)}}.
  
: '''On the more philosophical, [[epistemology|epistemological]] side:'''
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'''A posteriori knowledge''' are things that can only be knowable, or verifiable, from empirical evidence.
  
'''A priori''' generally means deduction from facts, or from first principles, from definitions, from agreements, towards consequences/effects.
 
Something is knowable a priori if it can be known independent of experience {{comment|(other than the language to communicate it)}}.
 
  
In many cases, a posteriori basically means "from logic" or "by definition".
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A priori knowledge often means you can reason it from principles or definitions.  
  
For example, "All bachelors are unmarried" or "If George V reigned for four years, that is more than three years," are a priori knowledge in that they depend purely on definition, not on evidence.
 
It is tautological and need only be pointed out to be useful.
 
  
{{comment|Some philosophers argue that a priori knowledge does not exist, or only ''very'' scarcely, e.g. that only self-contained definitions such as mathematical proof can qualify, and anything else we talk about is essentially a posteriori.}}
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That sentence makes it really easy to be weasely about it, through,
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because a lot of principles and definitions didn't come from nowhere.
  
  
  
'''A posteriori''' tends to refers to theories from experimental or subjective conclusions - from the particular to the general. Relatedly, often from effects to summaries or theorized causes.
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More practically:
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: You might still want to verify it using emperical evidence
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: a priori tends to mean we are taking general princtiples/definitions/agreements {{comment|(in part just because a lot of knowledge exists in the form in generalisations)}} and doing something useful with it, involving more specific consequences/effects
  
In many cases, a posteriori is almost synonymous with "empirical".
 
  
"Bachelors tend to be happy" or "George V reigned from 1910 to 1936" or "It is now raining outside" is something you cannot express from reason, it can only become knowable, or verifiable, by empirical evidence.
 
  
  
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In many cases, a posteriori is almost synonymous with "empirical",
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because it often goes from particular observations
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to more generalized descriptions or theories.
  
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"Bachelors tend to be happy" or "George V reigned from 1910 to 1936" or "It is now raining outside" is something you cannot arrive at from reason, because it came from observation and/or requires emperical verification.
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===A bit more practically===
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: '''In more pragmatic settings'''
 
: '''In more pragmatic settings'''
  
 
A priori tends to translate as 'pre-existing' or, as in prior.
 
A priori tends to translate as 'pre-existing' or, as in prior.
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This is where people weasel in extra meanings - some of them quite useful.
 
This is where people weasel in extra meanings - some of them quite useful.
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In a setting like everyday scientific discussion, a priori knowledge tends to mean "givens without (often ''before'') further inquiry".
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In a setting like everyday scientific discussion,  
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: a priori knowledge tends to mean "what is given without/before further inquiry"
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: a posteriori tends to mean "what we can conclude from this experiment"
  
The first blurs the line between logic and evidence, as it typically refers to things we have verified enough to be considered all but proven.
 
  
The second involves time into something which in the above, epistemological, sense is independent from time.
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This is not as clear-cut as it looks.  
  
  
In short, these uses may be useful in the everyday setting of figuring out things from data, but they also blur the line between priori and a posteriori.
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Note, however, that "what we know already" can easily blur the line between logic and evidence.
  
(Or at least shove their meaning in the direction of "stuff I've done" and "hypotheses I want to verify" respectively)
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Particularly in science, because philosophically speaking, science is all a posteriori,
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being about the best explanations of evidence.
  
For, arguably, reasons similar to what philosophers say when they say a priori knowledge does not exist, just in a more everyday "everything ''interesting'' is a posteriori" way.
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Practically, you could say the things is science that are a priori are called mathematics.
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Everything ''interesting'' to the everday will be a posteriori.
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(math sorta cheats anyway, by only caring about internal consistency, not the real world)
  
  
  
Note that coming before or after is often ''either'' about timing, ''or'' about cause.
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Since you can argue a priori barely exists, it's not a useful term, or distinction at all,
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it's been co-opted into softer distinctions.
  
"Smoking causes cancer" is a statement implied to be relevant always and everywhere,
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For example, in everyday science it's easy to say "you know that theory that's looked solid for the last hundred years? Let's treat that as fact, i.e. a priori knowledge, for this particular experiment".
so it was an a posteriori proposition before people considered the idea,  
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and will still be when everyone assumes it to be presumable/obvious/true.  
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Or even to roughly split into "stuff I've finished checking" and "hypotheses I want to look at.
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: '''A priori used to mean "previous evidence"'''
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: '''A priori as in "settled in the past"'''
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Note that 'coming before or after' can also involve timing, or cause.
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There is a similar co-opting of the terms going on here.
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Most of what we consider everyday facts, a priori(-ish), are really neither of these things,
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but it's still useful to treat them that way.
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Before people considered the idea, "Smoking causes cancer" was an proposition,
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a posteriori in that it was based on observations that cancer seemed to happen more in smokers.
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Now that evidence has shown this link to be pretty damn convincing,
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the ''next'' discussion could choose to consider it a priori knowledge,
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in the meaning of "stuff that came before, that we are reasoning from"
  
...but if it's so thoroughly proven, then the next discussion could consider it a priori knowledge,
 
in the meaning of "stuff that came before that we are reasoning from"
 
  
  
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This fuzzes the difference somewhat, because here, "a priori knowledge" is used in a "what we already know about this empirical information" sense.
 
This fuzzes the difference somewhat, because here, "a priori knowledge" is used in a "what we already know about this empirical information" sense.
 
 
 
 
 
For example:
 
* "Smoking causes cancer" is a posteriori
 
 
 
  
  
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However, when your context is  
 
However, when your context is  
  
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Latest revision as of 18:58, 12 September 2019

This article/section is a stub — probably a pile of half-sorted notes, is not well-checked so may have incorrect bits. (Feel free to ignore, fix, or tell me)


Most generally

Basically, A priori roughly means something "(from) that which goes before".

Often used in a "prior to experience/measurement".

A posteriori "(from) that which comes after".

Often meaning after experience, or using said experience



Knowledge (philosophically)

While in general a priori can be translated as 'pre-existing', once you start saying 'a priori knowledge' , you trip into epistemology (a.k.a. 'what can we know'), and a bit of metaphysics.


A priori knowledge: are things that can be knowable independently of experience/evidence (pedantry: ...aside from the experience of the language to communicate it).

A posteriori knowledge are things that can only be knowable, or verifiable, from empirical evidence.


A bit more practically

Statistics

In probability and statistics, particularly statistical inference, a priori is the prior knowledge of a population. Basically, it is anything factual that we can use to improve our model. It is more than just estimations or limited recent measurements. (verify)


A priori probability http://en.wikipedia.org/wiki/A_priori_probability

Posterior probability http://en.wikipedia.org/wiki/Posterior_probability


Modelling

In machine learning and pattern recognition, and the models and math that backs it, a priori refers to factual/good/positive examples that make for supervised learning.

Without such a priori examples, the patterns would depend on data behaviour, clustering and such. (verify)


Philosophy

In philosophy, a priori refers to that which is independant of experience, something that follows from logic.

A posteriori is that which is deduced from epirical evidence, from experience, observation, or personal decision.

The distinction is related to objective versus subjective observation.(verify)


Law

In law, a priori refers to being based on hypothesis or deduction rather than experimentation.

It can still refer to subjective, semantic details: testimonials are automatically subject to a priori plausability - personal back knowledge. (verify)


Linguistics

In linguistics, a priori constructed languages are those that are created from scratch.

A posteriori constructed languages are those that mix and match from existing ones.


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