Is a priori knowledge possible?

[Platos stepchild]

(Nested quote removed.)

Why not?

The reason why astronomy isn't an a priori, is that astronomical facts/truths are synthetic and metaphysically/ontologically contingent/non-necessary, and such truths/facts can be discovered and known only on the basis of perception/observation rather than on the basis of (rational) intuition.

And yet, it's precisely on the basis of rational intuition that Medieval scholars understood and translated the skies

[Consul]

The reason why astronomy isn't an a priori, is that astronomical facts/truths are synthetic and metaphysically/ontologically contingent/non-necessary, and such truths/facts can be discovered and known only on the basis of perception/observation rather than on the basis of (rational) intuition.

And yet, it's precisely on the basis of rational intuition that Medieval scholars understood and translated the skies

I'm not sure I understand your statement.
The concepts of apriority and aposteriority are epistemological ones rather than methdological ones. To use Hans Reichenbach's famous distinction, they belong to the context of justification (or confirmation) rather than to the context of discovery (or invention, theory/model-construction or -formation). That is, they are about how a hypothesis, theory, or model is justified/justifiable (confirmed/confirmable) and known/knowable rather than about how it is invented, generated, or constructed. To call a theory or model a priori in the context of discovery would be to say that it is invented or constructed on the basis of pure intuition, imagination, contemplation, or speculation, i.e. without empirical data or input gained through observation or experimentation. But an "a priori invented" theory is certainly not the same as an a priori justified theory.
Astronomical theories and natural-scientific theories in general are not justifiable or knowable a priori.

[Toledoroy]

That's a lovely question which reminds me a friend of mine, who's completely crazy.
Knowledge must be the knowledge of something.
If knowledge is priori, then it is not the knowledge of something but knowledge of nothing.

For example, Donald Tramp knows that his health plan is much better than Obama care.
This is knowledge that isn't connected to any experience, but due to that, no one else can gain this knowledge. Which leaves our beloved Trump knowing something that no one else can know, and therefore, delusional.

Priori knowledge = delusion.

[Consul]

That's a lovely question which reminds me a friend of mine, who's completely crazy.
Knowledge must be the knowledge of something.
If knowledge is priori, then it is not the knowledge of something but knowledge of nothing.

??? Both a posteriori knowledge and a priori knowledge are propositional knowledge of something, viz. truths or facts.

For example, Donald Tramp knows that his health plan is much better than Obama care.
This is knowledge that isn't connected to any experience, but due to that, no one else can gain this knowledge. Which leaves our beloved Trump knowing something that no one else can know, and therefore, delusional.

This is not an example of genuine knowledge but of mere knowledge-belief or subjective certainty.

-- Updated April 16th, 2017, 11:51 am to add the following --

If the question of the general possibility of a priori knowledge (of truths/facts) is answered in the affirmative, then the next question is: what kinds of truths/facts (true propositions) are knowable a priori?

Here's a list of the possibilities:

1. analytic ones (all of which are logically necessary)
2. synthetic ones
2.1 necessary synthetic ones
2.1.1 metaphysically/ontologically necessary ones
2.1.2 nomologically necessary ones (laws of nature)
2.2 non-necessary/contingent ones

[Wayne92587]

read post#267

[Felix]

Consul: Astronomical theories and natural-scientific theories in general are not justifiable or knowable a priori.

Not justifiable without empirical evidence, but knowable (rationally intuited) long before the empirical evidence that confirmed the intuition was gathered (as was the case with certain aspects of Einstein's Theory of Relativity). I think that was Plato stepchild's point.

[Consul]

Consul: Astronomical theories and natural-scientific theories in general are not justifiable or knowable a priori.

Not justifiable without empirical evidence, but knowable (rationally intuited) long before the empirical evidence that confirmed the intuition was gathered (as was the case with certain aspects of Einstein's Theory of Relativity). I think that was Plato stepchild's point.

If it was, then he's wrong, because nobody can "intuit" the truth of a (natural-)scientific theory.

[Wayne92587]

Felix

Not justifiable without empirical evidence, but knowable (rationally intuited) long before the empirical evidence that confirmed the intuition was gathered (as was the case with certain aspects of Einstein's Theory of Relativity). I think that was Plato stepchild's point.

Many believe that most of Einstein's Theories are bogus.

-- Updated April 16th, 2017, 12:36 pm to add the following --

Einstein's curved Universe to especially bad.

Motion in Space-Time especially the motion of Light is curved because said Motion is geodesic in nature not because space is curved.

-- Updated April 16th, 2017, 12:55 pm to add the following --

When the Reality of First Cause, the First Singularity to have relative value, a numerical value of One-1 was created, due to displacement, the Motion of a Singular particle having no relative, numerical value, having a numerical value of Zero-0 was converted into a Singularity having relative a numerical value of One-1.

A Singularity in order to be relative, have a numerical value of One-1, must have been displaced, have angular momentum, velocity of speed and direction.
A Singularity having no relative, numerical value, having a numerical value of Zero-0 having no displacement has no angular momentum, has no velocity of speed and direction is dead in the water, does not move.

The motion of the First Singularity to have relative, a numerical value of One-1, the Reality of First Cause was the single direct cause of a system of Chaos (as in the Butterfly Effect) which made manifest the Heavens and the Earth, the Universe, the Reality of Every thing that exists in the material, physical, sense of the word.

The Theory as to the cause of the Reality of Everything can be can be explained by a mathematical formula, an equation. 0/1

[Toledoroy]

??? Both a posteriori knowledge and a priori knowledge are propositional knowledge of something, viz. truths or facts.

Yeah, I made a mess
I see what you mean, there is definitely knowledge that is completely abstract.
So I'm guessing priori knowledge would be the knowledge of definitions, classifications, and such?

[Wayne92587]

The difficulty is that some "believe," conjecture, that knowledge need not be experienced.

Reason can lead to irrationality, Illusion, an abstract reality, a subjective Reality.

Rationalization is Mankind's helpmate not a means to an end.

[Consul]

I see what you mean, there is definitely knowledge that is completely abstract.
So I'm guessing priori knowledge would be the knowledge of definitions, classifications, and such?

The (sharpness of the) distinction between analytic and synthetic propositions is a contentious issue, but one can fairly say that a proposition (statement/sentence) is analytic if and only if its truth(-value) is determined solely by its form(al structure) [syntax] or/and its meaning, the meaning of its component elements [semantics], such that its truth(-value) can be ascertained by purely logical, conceptual, or linguistic analysis. (Some would add "…or mathematical analysis".) And a proposition is synthetic if and only if it is not analytic.

According to radical empiricism, no—neither analytic nor synthetic—propositions are a priori, non-empirically knowable or justifiable.
According to moderate empiricism, only analytic propositions are a priori, non-empirically knowable or justifiable.

"analytic vs synthetic sentence. In philosophy, a distinction is traditionally drawn between analytic and synthetic statements or sentences. (a) Analytic sentences in the narrow sense (also logically true sentences) are statements that necessarily, i.e. in all possible worlds, are true solely on the basis of their logical form and whose truth can be determined without empirically checking it; cf. Either it's raining, or it's not raining. Analytic sentences in the broader sense are those whose truth depends on their syntactic structure and on the meaning of their linguistic elements. They are based on semantic relations such as semantic similarity (i.e. synonymy) and semantic inclusion (i.e. hyponymy); cf. the statement Siblings are related to one another. (b) Synthetic sentences, on the other hand, are those statements about relationships of facts whose truth depends not only on their syntactic or semantic structure. but on extralinguistic factors and experience and thus can be empirically checked; cf. Bill Clinton is the 42nd president of the United States. That is, while analytic sentences are necessarily true, synthetic sentences are true or false depending on the composition of the world described by them."

(Bussmann, Hadumod. Routledge Dictionary of Language and Linguistics. Translated and edited by Gregory P. Trauth and Kerstin Kazzazi. London: Routledge, 1996. pp. 57-8)

"‘Analytic’ has been used in a wide variety of ways: truth by conceptual containment and truth whose denial is contradictory (Kant 1781/1787); logical truth (Bolzano 1837; Feigl 1949); truth by definition and logical derivation (Frege 1884; Pap 1958); truth in virtue of form (Schlick 1930–1); truth by definition and logical truth (Carnap 1937, 1947); truth by definition (Ayer 1936); truth based on meaning (Ayer 1936; C.I. Lewis 1944); truth by semantical rule (Carnap 1947); truth in all possible worlds (C.I. Lewis 1944; D.K. Lewis 1969); convertibility into logical truth by substitution of synonyms (Quine 1951); truth by implicit convention (Putnam 1962); and so on. Although related, not all of these uses are equivalent. For example, logical truths are not true by definition (in the sense of explicit definition), but they are trivially true by definition plus logic. Furthermore, Gödel’s incompleteness result shows that logical derivability and logical truth are not equivalent. Likewise various principles (for example, supervenience principles) which are true in all possible worlds seem not to be true by definition plus logic (if ‘definition’ does not include ‘implicit definitions’). Similarly, it may be doubted that correct definitions provide exact synonyms. Little care has been taken to distinguish these disparate uses and needless confusions have resulted. But, as Strawson and Grice (1956) note, observations of this sort ‘would scarcely amount to a rejection of the distinction [as Quine urges]. They would, rather, be a prelude to clarification’."

(Bealer, George. "Analyticity." In The Routledge Encyclopedia of Philosophy, vol. 1, edited by Edward Craig, 234-239. London: Routledge, 1998. pp. 234-5)

[Felix]

o.k., Einstein was a poor example of "a priori" scientific knowledge because he was a modern man with scientific training. Copernicus would be a better example. He rationally intuited astronomical ideas that could not be empirically verified until several decades after his death - the instruments needed to verify his concepts did not exist when he was alive.

[Consul]

o.k., Einstein was a poor example of "a priori" scientific knowledge because he was a modern man with scientific training. Copernicus would be a better example. He rationally intuited astronomical ideas that could not be empirically verified until several decades after his death - the instruments needed to verify his concepts did not exist when he was alive.

I already mentioned the distinction between the methodological (and psychological or sociological) context of invention and the epistemological context of justification. To come up with an idea or to devise a theory or model intuitively ("instinctively") in the context of invention is not thereby to be justified in believing or to know that it is true on the basis of (rational) intuition. An intuitively generated idea is not the same as an intuitively justified idea.

[Wayne92587]

Priori Knowledge is not the Knowledge of an objective, empirical, Reality, is not born of experience, is a discovery derived through the indirect method of attaining the knowledge of reality, Prior Knowledge is gleaned from the Cosmos, many sources.

[Felix]

Consul: To call a theory or model a priori in the context of discovery would be to say that it is invented or constructed on the basis of pure intuition, imagination, contemplation, or speculation, i.e., without empirical data or input gained through observation or experimentation.

That is what I was referring to: intuitive knowledge not gained via empirical analysis or experimentation. You yourself listed it as a possible apriori truth: 2.1.2 nomologically necessary ones (laws of nature).