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Due to this post being lost in the handover from SIN to PAtheos, I am reposting it for the records, to add to the list of other philpapers ones. This one is a shorter one than many of the others.

So having posted the Philpapers survey results, the biggest ever survey of philosophers conducted in 2009, several readers were not aware of it (the reason for re-communicating it) and were unsure as to what some of the questions were. I offered to do a series on them, so here it is – Philosophy 101 (Philpapers induced). I will go down the questions in order. I will explain the terms and the question, whilst also giving some context within the discipline of Philosophy of Religion.

The first question is “a priori knowledge: yes or no?”

a priori

As one can guess from the use of the word for ‘prior’, this is knowledge that is ‘prior to’ evidence (or experience). In other words, it does not require evidence to justify it as knowledge, in effect being self-evident. There are questions as to whether such knowledge can be defeated by empirical evidence, and whether it must be necessary (or knowledge in all possible worlds). Necessary propositions cannot be false, such as ‘all sisters are female’. There is certainly a large element of semantics and definitional language here. This includes tautologies such as ‘all bachelors are unmarried’, which is true by definition (in a slightly different manner to the previous proposition).

This might lead one on to claim that language is a desciptive layer added on to observed phenomena (over time).

There is certainly an element of intuition with a priori knowledge. Rationalists are seen as the set of philosophers who adhere to the coherence of a priori knowledge.

Maths, for example, seems to have elements of a priori justification. That 6+4=10 is intuitively rational. Deductive arguments in logic also assume this. Whether intuition is enough to justify knowledge is, it appears, the nub of this philpapers question. Are logic and maths, for example, descriptive languages applied to reality after observing reality? Or are maths and logic conceptually pre-existing phenomena which we, as humans, merely access?

Rationalists can be seen as opposing empiricists in that they argue for the primacy of thought, ideas, concepts and a priori knowledge, whereas empricists see knowledge as dependent on observation and evidence.

The Stanford Encyclopedia of Philosophy sums things up well, as ever, in defining the argument between rationalists and empiricists:

Rationalists have typically thought that we can be a priori justified, and even know, things about the world, and empiricists have denied this. Now if the world includes abstract entities like numbers and propositions, then some rationalists, and even some empiricists, will hold that we can know a priori things about the existence and nature of these entities (though the empiricists might have a different view about what it is to be an abstract entity). However, rationalists like BonJour (1998) will insist that we can also know a priori things about the natural world. For instance, we can know a priori that no object can be red and green all over at the same time and in the same respects, that no object can be wholly in two distinct places at the same time, and (perhaps) that backward causation is impossible. They will claim that this is knowledge of the nature of reality and will be true of any object, or event, that exists.

One might grant this claim and at the same time point out that it does not give us knowledge of the existence of things, events, and states of affairs but only knowledge of what they must be like if they exist. We only know that there are objects and events in space and time by experiencing them, even if we can know a priori certain things about the distribution of colors on their surfaces, how many places they can be in at any given time, and whether a later event can cause an earlier one.

It does, obviously, depend on how you define knowledge. In a Cartesian sense, the only thing we can indubitably know is cogito ergo sum, that “I” exist. Everything else has an element of doubt. Perhaps we are brains in vats. Our minds are experiencing machines, and in a sense, all experience of the world is sense data. Does this include logic and the things that are often deemed a priori?

There are many objections, and counterpoints to those objections (the notion that reason, or reflection alone, can justify knowledge) concerning these ideas, and if you want a deeper analysis of the world of a priori, then hit the link above.

It would be rude not to include the other ‘a’s, in terms of knowledge, here.

A posteriori

Simply put, empirical evidence (meaning ‘from the later’). There, that was easy. This is empirical data as well as (or which indeed is) sense experience. When we talk about logical syllogisms, deductive arguments apply more to a priori style propositions, whilst inductive arguments apply a posteriori.


1)      All men are mortal

2)      Socrates is a man

3)      Therefore, Socrates is mortal

Is deductive a priori, whilst:

1)      All swans that I have seen are white

2)      Therefore, all swans are white


1)      All swans I have seen are white

2)      Therefore, in all probability, the next swan I will see will be white

Now, we could confuse matters here by claiming that the first few premises of the deductive, a priori argument, are in fact inductive conclusions such that:

1)      All men that I have seen are mortal

2)      Socrates is, according to my sense data, a man

3)      Therefore, Socrates is mortal

Which opens a whole can of worms. Perhaps this then leads us to conclude that all knowledge is, at base, inductive…?

A fortiori

This term is less well-used. An argumentum a fortiori denotes “argument ‘from [the] stronger [reason]’ which entails the use, usually, of probability. When faced with two propositions, arguing for the one which is probabilistically more likely would be such an argument. Using Bayes’s Theorem would be an obvious method to employ in a fortoriori arguments. Examples here.

Just finally, let me know if this is useful, and spark a debate about whether you think a priori makes sense, or whether our foundations of knowledge are a posteriori.


#1 – a priori

#2 – Abstract objects – Platonism or nominalism?

#3 – Aesthetic value: objective or subjective

#4 – Analytic-Synthetic Distinction

#5 – Epistemic justification: internalism or externalism?

#6  – External world: idealism, skepticism, or non-skeptical realism?

#7 – Free will: compatibilism, libertarianism, or no free will?

#8 – Belief in God: theism or atheism?


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Jonathan MS Pearce

A TIPPLING PHILOSOPHER Jonathan MS Pearce is a philosopher, author, columnist, and public speaker with an interest in writing about almost anything, from skepticism to science, politics, and morality,...