Difference between revisions of "A priori, a posteriori"

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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.

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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