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In this paper I will offer a novel understanding of a priori knowledge. My claim is that the sharp distinction that is usually made between a priori and a posteriori knowledge is groundless. It will be argued that a plausible understanding of a priori and a posteriori knowledge has to acknowledge that they are in a constant bootstrapping relationship. It is also crucial that we distinguish between a priori propositions that hold in the actual world and merely possible, non-actual a priori propositions, as we will see when considering cases like Euclidean geometry. Furthermore, contrary to what Kripke seems to suggest, a priori knowledge is intimately connected with metaphysical modality, indeed, grounded in it. The task of a priori reasoning, according to this account, is to delimit the space of metaphysically possible worlds in order for us to be able to determine what is actual.

The notion of the a priori underwent several changes since the time it came into existence in the Middle Ages. Originally it had been used to mark a certain form of argument, an argument that proceeds from what is prior to what is later, from cause to effect: demonstratio procedens ex causis ad effectum = demonstratio a priori. But this changed with Kant, for whom it meant not a form of argument but rather some special kind of knowledge (or elements thereof), namely knowledge that (a) is independent of particular experiences and (b) that makes experience in general (Erfahrung überhaupt) possible. Tied up with consciousness and the transcendental unity of apperception, Kant"s understanding of the a priori was in the spirit of his transcendental philosophy. But this understanding changed again with the rise of analytic philosophy, in which we still find the first characteristic but not the second anymore. The idea of Erfahrung überhaupt was given up, partly because one naturally wondered what exactly this notion of experience in general, or experience universally conceived, should be. Where should we get it from, if not by way of abstraction and generalization from individual cases of experience? And would this not make it an empirical concept, so that the whole project of asking for the conditions of its possibility would not lead us to the kind of certainty, necessity and universality we expect from a priori knowledge? There would be no guarantee that in the future we would not make discoveries that would give us new kinds of experiences or that would show us our experiences in a new light. Thus, we would have to admit that these experiences did not satisfy the conditions of experience we had set up originally. The a priori conditions would have to be revised.

If the a priori is the proper subject matter of transcendental philosophy, then the problems of the a priori are also problems for transcendental philosophy. the idea that defines transcendental philosophy is the idea that there are stable general structures which are discernible in experience, provide the foundations of our knowledge of it, and collectively constitute an a priori which transcends experience and informs it. the a priori is traditionally conceived as a nexus of relations which is held to be logically/temporally prior to experience and responsible for its organization. in this essay, i challenge this traditional (kantian) understanding of the a priori and set forth an alternative to it based on the writings of maurice merleau-ponty. the intent is to provide a new conception of transcendental philosophy.

The distinction between a priori and a posteriori knowledge has been the subject of an enormous amount of discussion, but the literature is biased against recognizing the intimate relationship between these forms of knowledge. For instance, it seems to be almost impossible to find a sample of pure a priori or a posteriori knowledge. In this paper, it will be suggested that distinguishing between a priori and a posteriori is more problematic than is often suggested, and that a priori and a posteriori resources are in fact used in parallel. We will define this relationship between a priori and a posteriori knowledge as the bootstrapping relationship. As we will see, this relationship gives us reasons to seek for an altogether novel definition of a priori and a posteriori knowledge. Specifically, we will have to analyse the relationship between a priori knowledge and a priori reasoning , and it will be suggested that the latter serves as a more promising starting point for the analysis of aprioricity. We will also analyse a number of examples from the natural sciences and consider the role of a priori reasoning in these examples. The focus of this paper is the analysis of the concepts of a priori and a posteriori knowledge rather than the epistemic domain of a posteriori and a priori justification.

1. Background. At least from the time of the ancient Greeks, most philosophers have held that some of our knowledge is independent of experience, or “a priori”. Indeed, a major tenet of the rationalist tradition in philosophy was that a great deal of our knowledge had this character: even Kant, a critic of some of the overblown claims of rationalism, thought that the structure of space could be known a priori, as could many of the fundamental principles of physics; and Hegel is reputed to have claimed to have deduced on a priori grounds that the number of planets is exactly five.

The terms "a priori" and "a posteriori" refer primarily to how or on what basis a proposition might be known. A proposition is knowable a priori if it is knowable independently of experience. A proposition is knowable a posteriori if it is knowable on the basis of experience. The a priori/a posteriori distinction is epistemological and should not be confused with the metaphysical distinction between the necessary and the contingent or the semantical or logical distinction between the analytic and the synthetic. Two aspects of the a priori/a posteriori distinction require clarification: the conception of experience on which the distinction turns; and the sense in which a priori knowledge is independent of such experience. The latter gives rise to important questions regarding the positive basis of a priori knowledge.

The book sets out to analyse the notion of a priori justification and of a priori knowledge.

One might think that a healthy respect for the deliverances of experience would require us to give up any claim to nontrivial a priori knowledge. One way it might not would be if the very admission of something as an episode of experience required the use of substantive a priori knowledge -- if there were certain a priori standards that a representation had to meet in order to count as an experience, rather than as, say, a memory or daydream. This paper argues that, surprisingly enough, we can find elements of this essentially Kantian line about experience even in the work of empiricists, such as John Locke and Bas van Fraassen.

Kant''s claim that the justification of transcendental philosophy is a priori is puzzling because it should be consistent with (1) his general restriction on the justification of knowledge, that intuitions must play a role in the justification of all nondegenerate knowledge, with (2) the implausibility of a priori intuitions being the only ones on which transcendental philosophy is founded, and with (3) his professed view that transcendental philosophy is not analytic. I argue that this puzzle can be solved, that according to Kant transcendental philosophy is justified a priori in the sense that the only empirical information required for its justification can be derived from any possible human experience. Transcendental justification does not rely on any more particular or special observations or experiments. Philip Kitcher''s general account of apriority in Kant captures this aspect of a priori knowledge. Nevertheless, I argue that Kitcher''s account goes wrong in the link it specifies between apriority and certainty.