While the practice of human rights courts promotes coherence among the regional and the universal human rights regimes, the principle of systemic integration is not meant to expand the normative content of the interpreted treaty on the basis of external instruments, especially non-binding instruments. Therefore, such practice cannot be conceptualised as interpretation, but as the application of external instruments. To defend this argument, I rely in the theory of Alf Ross concerning the sources of international law. Then, after adjusting Ross’s theory to the specific problems of the 21st century, and proposing three mutually reinforcing notions (specificity, completeness and purpose) that assist the judge in determining the applicable law to a case, I develop the content of the normative plurality hypothesis.