Hello. Thank you for reading this. I'm in grave need of philosophical counsel please. I cannot 'get' the distinction between 'a priori' and 'a posteriori'. It seems to me that anything that is known must be, in some way, related to experience. I'm troubled by this thought experiment: If a baby was born with a terrible genetic condition which excluded all the human senses, what would the child 'know'? Without the 'experience' of the senses, what could the child ever know? Not even syllogism would be possible; without experience, language would not be available to the unfortunate child. And I imagine that this would be true of numbers too. Yours truly, Blunderov.

Consider the following quotation from Kant:

There can be no doubt that all our knowledge begins with experience. […] [I]t does not follow that it all arises out of experience. (Critique of Pure Reason, B1)

The point being made is that, of course, your unfortunate child would have nothing to think about without sensory input. But nevertheless, given experience, it may be that the mind makes an a priori contribution to that experience.

By the way, I must disagree slightly with my colleagues writing below, at least as far as the interpretation of Kant . From the point of view of Kant, the justification account in Frege oversimplifies matters. This for three reasons. First, because on Kant's analysis there must be at least two distinct a priori grounds, neither of which can ground the other (sensibility and understanding). Thus, for Kant, we must not conflate the distinctions a priori/ a posteriori and analytic/ synthetic. Second, because an analysis of these grounds can (Kant argues) provide a science of metaphysical propositions; that is, there is specific a priori content. Third, Kant leaves room for impure a priori knowledge, and the famous principle of causation is included here because 'change' is an empirical concept (this is stated a couple of paragraphs after the quotation above). There is, for Kant, a kind of necessary interplay between that which is empirically given and the a priori, which is what I meant above in saying 'contribution'.

At about the same time as Frege, the philosopher Husserl was interpreting Kant in roughly this way, and developing what he called 'phenomenology'.

Not surprising, then, that someone might not 'get' the distinction, since there continue to be disagreements about it!

To answer your good question, one needs to distinguish between the role experience plays in the acquisition of knowledge and the role it plays in the justification of knowledge. You're absolutely right that without experience humans would not be able to develop cognitively; they would not be able to acquire knowledge of anything at all. So experience, like oxygen, is needed in order for us to become knowers. But to say that some proposition is known a priori is not in conflict with this claim. To say that some proposition is knowable a priori means that one's justification of the claim need make no reference to information obtained through the senses. We can justify Pythagoras' Theorem without any information provided by our sense organs (take a look at any proof of it and you'll see that this is so). Hence, it is knowable a priori – even though no one could have known it unless they had had the experiences needed in order for their minds to develop.

Here's Frege's way of making this point:

Now these distinctions between a prioir and a posteriori, synthetic and analytic, concern, as I see it, not the content of the judgement but the justification for making the judgement. ...When a proposition is called a posteriori or a priori in my sense, this is not a judgement about the conditions, psychological, physiological and physical, which have made it possible to form the content of the proposition in our consciousness; nor is it a judgement about the way in which some other man has come, perhaps erroneously, to believe it true; rather, it is a judgement about the ultimate ground upon which rests the justification for holding it to be true. (Foundations of Arithmetic, section 3)

Frege attaches a footnote in the middle of the first sentence in which he says that he means "only to state accurately what earlier writers, Kant in particular, have meant by" these terms.

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