In this article, I discuss the life and work in Croatian historiography of the completely neglected but at the time prominent Zagreb physician Dr. Karl Marchesi, both in respect to his orthodox medical career as an internist and pioneer of medical hypnosis in his homeland during the 1920s and 1930s and then in the United States, where he emigrated in 1950 and remained until his death, and his heterodox career as one of the restorers and main protagonists of the spiritist movement and leaders of parapsychological research first in Croatia and then in the United States as a close collaborator of Joseph Banks Rhine at the Parapsychological Laboratory at Duke University, with whom he established a collaboration already from his homeland in 1938. The article is divided into two parts, and in this first part I reconstruct the rich biography of Dr. Marchesi and analyze his role in the Croatian spiritist movement during the 1930s. In this analysis, particular attention is paid to the broader socio-historical context of the nature and genesis of the spiritist movements, especially in Croatia, of which Dr. Marchesi was a typical exemplar, and which is by itself also a fairly neglected topic of the histories of science, philosophy, and theology in Croatia. The second part of the article will discuss Dr. Marchesi’s heterodox work in parapsychology and hypnotism, the latter having grown from a controversial to a legitimate scientific discipline of medical hypnosis in which Dr. Marchesi has left a noticeable international mark. Also, in the second part of the article, I will reflect on the general relationship between ‘orthodoxy’ and heterodoxy’ in science, as well as on the relationship between science and pseudoscience in light of the relevant knowledge of modern history and philosophy of science, as opposed to some ingrained but erroneous narratives about this relationship.