Non-empirical principles have always been a subject of interest of philoso-phers. Authors from different times and traditions agree that principles such as analogy or simplicity are present in the scientific practice. The disagreement comes out when these authors affirm that these principles have an epistemic function, and when they try to present reasons in order to found this state-ment. The first goal of this paper is to describe these principles and to point out their methodological importance. The second goal is (...) to introduce the discussion between the two most important models of philosophical fundamentation – it means, the logicism and the psychologism. The last goal – the main one – is to defend that the non-empirical principals have, legitimately, an epistemic role in the scientific investigation. (shrink)