In his famous treatise "Theory of Knowledge"the  German philosopher J. Hessen wrote:

"The history of philosophy has a second attempt to mediate between rationalism and empiricism: apriorism. This last one also considers the experience and thought as sources of knowledge. But apriorism defines the relationship between experience and thought in a sense directly opposed to intellectualism. As the name apriorism says, our knowledge has, in this sense of direction, elements a priori, independent of experience. This was also the view of rationalism. But as this one considered a priori factors as contents, as perfect concepts for apriorism these factors are of a formal nature. They are not content, but forms of knowledge. These forrms receive the contents from the experience, and in this apriorism separates from rationalism and approaches to empiricism"(J.Hessen, Theory of Knowledge, Espasa Calpe Argentina, Buenos Aires-Mexico, Third Edition, 1944, p. 52 , as highlighted in bold is placed by us).

Hessen is wrong, like the generality of specialists in epistemology. There is no distinction between nativist rationalism and  apriorism to the extent that the last is a modality of the second: the theory that man at birth already has innate ideas (nativism) is a substantial apriorism. There is no distinction between empiricism and formal apriorism to the extent that the first is incorporated in the second: almost all empiricists are formal apriorists since argue that there are formal structures a priori as the organs of sense and reason but destitute of contents.

Kant's theory is a substantial apriorism, since the ideas or concepts of necessity, unity, plurality, triangle, circle, numbers one, two, three, etc. are innate, and exist in sensitivity (shapes and numbers) as is in the understanding a priori (categories: unity, plurality, necessity, divisibility, etc.). And it is a rationalism because holds that reason, in its lowest form of understanding, is the main, but not exclusive, constructor of knowledge.

Apriorism has his contrary in aposteriorism: both they belong to the genus «temporalizre».  It  is not opposed to rationalism or empiricism: both of these ones belong to the genus «origin of knowledge». Hessen missed, the same as Kant, a true dialectical thinking.

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