Some questions about Linguistics, Psycholinguistics and Neurosciences interfaces are presented and discussed, specially the ontological incommensurability and the granularity problem (Poeppel and Embick, 2005), which gains new outlines when they are analyzed under a Reduction Problem of the Philosophy of Mind perspective (Heil, 1998; Teixeira, 2000). From this point of view, both problems can be investigated from the solutions of this problem which were formulated in the identity theories, reductionism, functionalism and supervenience philosophical theories fields. The relevance of these proposals is evaluated concerning incommensurability and the granularity, whose study is considered fundamental in the research of possible interfaces between language sciences.