The article seeks to illustrate how Mikel Dufrenne’s realist phenomenology develops a theory and classification of material a priori designed both to provide a foundation for individual sciences and to elaborate a transcendental philosophy. Since such a priori are symmetrically present in both the subject and the object, they fulflll a transcendental or constitutive function in each, making it possible to highlight the “transcendental specificities” of each dimension under analysis. Through this hypothesis, the philosopher seeks to distance himself both from transcendental idealism and from naïve realism.