In fin-de-siècle France, we witness a strange circulation of concepts between philosophy, theoretical and experimental psychology, and the borderline realm of what we would now call meta- or parapsychology. This was a time characterized by a complex process of redefinition of the disciplinary frontiers between philosophy and psychology, which favoured the birth of hybrid conceptualities and stark oppositions as well. Furthermore, the great scientific advances in physics, physiology, and psychology fostered hope for a full rational explanation of reality, even of (...) its most unfathomable layers and seemingly bizarre phenomena. Focusing on the case of Émile Boirac’s research on what he termed ‘cryptopsychism’, notably in his book Our Hidden Forces, this article aims to show how Kantian notions and models of consciousness belonging to the canon of French spiritualist philosophical psychology were taken up by scientists such as Pierre Janet and ended up being assimilated and discussed in the more obscure and precarious realm of scientific inquiry into metapsychical phenomena. Far from being a mere historical curiosity, this quest for a scientific account of the latent and subconscious life of the mind sheds light on the intricate relationship between philosophy and the human sciences between the 19th and 20th centuries. (shrink)