Definition of
   a priori






Scepticism (or US: Skepticism) is a philosophic orientation that questions existence of "absolutely certain" arguments.

It probably grew as a reaction against demagogues and priests that came up with stories that gave them power, sex, incomes and/or cultural status.


For it is not the Roman clergy only, that pretends the kingdom of God to be of this world, and thereby to have a power therein, distinct from that of the civil state.

Hobbes 1651 - Leviathan 4.47 (English Works vol.03, Molesworth, Bohn 1839, p.700).


Extreme scepticism is untenable


An extreme sceptic would claim that knowledge does not exist. It would then die within a week, e.g. because it has not been drinking.

It would not have listened to the knowledge that water is important for our survival.

An extreme sceptic is called Pyrrhonist after Pyrrho (about 300BCE). It may try to justify its philosophy in words, but have no possibility to do so in real life.

When a cur /mad dog/ rushed at him and terrified him, he answered his critic that it was not easy entirely to strip oneself of human weakness; but one should strive with all one's might against facts, by deeds if possible, and if not, in word.

Diogenes Laertius (about 200-250 CE) - Lives of Eminent Philosophers vol.02 (Hicks, Loeb Class. Lib. #185, Heinemann 1925, p.479).

Not a sceptic founding thesis

A Pyrrhonist that claim nonexistence of knowledge makes a logical mistake because it claims that knowledge about this non-existence actually exists.

Plato's Socrates committed such a logical mistake when he praised himself:

I am wiser than this man; it is likely that neither of us knows anything worthwhile, but he thinks he knows something when he does not, whereas when I do not know, neither do I think I know.

Plato - Apology, 21d.


Balanced scepticism


A milder and realistic flavour of scepticism was developed at Plato's Academy under Carneades (about 214–129 BCE), or possibly already under Arcesilaus (about 316-240 BCE).

They claimed that some arguments were more probable than others [Sextus], [Cicero].

This is a very important epistemological thesis, since it implies that some statements may be probable to an extent of being "very probable", i.e. being what we in ordinary speech call "true".

It is important because it provides a description of the term "true", and through this of "knowledge", that is not founded on improbable arguments.

Within balanced or mitigated scepticism, which also is called academic from Plato's Academy, it is claimed that our understanding ultimately is created from perception. From these premises we may through reasoning create very probable, i.e. trustworthy, opinions.

… probability is the very guide of life

Butler 1736 - Analogy of Religion, Harper & Brothers (1861), Introduction, p.84.

David Hume was an advocate of balanced scepticism, but not (which is too often claimed) extreme scepticism.


Hume proposed that very probable or trustworthy arguments should be termed as proofs:

Mr. Locke divides all arguments into demonstrative and probable.
In this view, we must say, that it is only probable all men must die, or
that the sun will rise to-morrow.

But to conform our language more to
common use, we ought to divide arguments into demonstrations, proofs,
and probabilities. By proofs meaning such arguments from experience
as leave no room for doubt or opposition.

Hume 1777 - Enquiry, intro, footnote sect.6.

Hume's demarcation leads into problems and I therefore use the phrase "very probable argument", and not "proof".

What we experience as "knowledge" is hence "very probable" arguments, created through reasoning from premises that ultimately has become known through perception.

This was claimed already by Aristotle:

Thus the states /of understanding/ neither belong in us in a determinate form, nor come about from other states that are more cognitive; but they come about from perception.

Aristotle - Posterior Analytics 2, §19, 100a10.




Balanced scepticism forms the philosophic basis for empiricism and scientific methodology.

Empiricism is the thesis that there is no a priori metaphysical knowledge and all concepts are derived from experience.
Empiricism therefore entails that there is no knowledge unless there is empirical knowledge.

Priest 2007 - The British Empiricists 2Ed, p.5.

the view that /.../ all rationally acceptable beliefs or propositions are justifiable or knowable only through experience.

Encycl. Britannica 2009 - Empiricism

An empiric hence claims that trustworthy arguments are created exclusively through reasoning that is ultimately based on perception.

Empiric conclusions are hence formed from perception or from reasoning where the premises are based on perception or from earlier reasoning where the premises ultimately are based on perception


All true sciences are the result of experiences which have passed through our senses.

Leonardo da Vinci (1452-1519) - Note in Trattato della Pittura, Vatican Library (Codex Urbinas 1270), 33r [Hendrix].

Not empiric founding thesis

The empiric founding thesis shows a similar illogical opinion as is seen for extreme sceptics. It claims that all argument ultimately should be based on perception in order to be trustworthy.

But it is impossible to verify "all" arguments, the thesis is only applicable to those that are already known and inferred.

The thesis has, however, never been falsified in a reliable manner and has directed humanity's focus towards observed reality that has resulted in an enormous growth of knowledge, economy and social progress.


Empiricism vs. balanced scepticism

The conceptions within empiricism and balanced scepticism are quite similar.

The main difference is that the empiric denies existence of "absolutely certain" or a priori knowledge, while the balanced sceptic embrace the logically more attractive opinion of staying passive in this question

My opinion is that an empiric denies credibility of a speaker that claims a basic argument that is neither verified nor is comparable with previously verified phenomena.

In case an a priori argument would be verified in a trustworthy manner, it would be accepted as empiric. The difference between the two opinions is hence in reality quite small.

The problem is, however, that an attempt to demonstrate existence of such an argument cannot refer to observations. Hence, if the founding thesis of empiricism is correct, they cannot refer to anything.


Empiricism without traceable verification becomes confused

In a manner similar to when non-verified arguments are spread today, justified by "several types of knowledge exist", an arbitrary statement may be claimed to be empiric.

Before scientific methodology was widely accepted, empirics existed that behaved like what we today call relativists:

But the word Empiric is now more odious than ever being confounded with that of Charletan, or Quack, and applied to persons who practise Physic at random, without a proper Education, or understanding any thing of the Principles of the Art. See Charletan.

Chambers 1728 - Cyclopaedia, term: EMPIRIC or EMPIRICK.


The success of Empiricism


The success for Empiricism is hence not solely due to propositions about observed phenomena. Such propositions have always been claimed, e.g. by prophets and deceivers.


It is the insistence on that propositions about phenomena should be possible to verify, i.e. scientific methodology, that has established the fantastic human progress that is said to be founded on empiricism.

Cicero - Lucullus 2.99.
Hendrix & Carman (eds.) - Renaissance Theories of Vision, Ashgate 2010.
Sextus Empiricus - Outlines of Pyrrhonism, Book 1, ch.220-227.