Plato’s Timaeus is the basic text on which relies Herman when formulating the key positions of his cosmology exposed in his work De essentiis. The only source of knowledge of Plato’s dialogue Timaeus for Hermanus is the translation made by Calcidius toward the end of the 4th century. Calcidius accompanied his translation with the commentary. His translation and commentary relate only to part of the Timaeus from 17A to 53C. In our article we are outlining the similarities and differences in Calcidius’s and Herman’s attitudes. Particular attention is paid to the differences, with the emphasis on the role of astrology and hermeticism in Hermann’s philosophy. The paper pays particular attention to hermeticism, which, in the opinion of those who interpreted the philosophy of Herman, played a significant role in his philosophy. Ch. H. Haskins for example, argues that Hermes was the highest authority for Herman. On the basis of such views arises first of all the question of whether the hermetic element played really such an important role in Hermann’s philosophy. In order to answer that question in the paper we analyze all the places in De essentiis where Herman is refering to Hermes. Then we examine the sources of Herman’s knowledge of the Hermetic writings. In two chapters we elaborate the reception of these writings in Latin and Arabic writers. In the final part we come to the conclusion that Herman knew the writings attributed to Hermes from Latin and from Arab sources. In the West, namely, starting from the Fathers of the Church, there was present the continuous reception of one of the Hermetic writings, Asclepius, the writing that belongs to a so-called learned hermeticism (that is in accordance with Festugière’s division of the Hermetic writings in the writings of learned and of popular hermeticism), from which Herman in his De essentiis draws most of the citations relating to the Hermetic writings. Hermes was a also a great authority for the representatives of the school of Chartres, with whom Herman was closely connected. But Herman’s main source of knowledge of the hermetic tradition were the writings of Arab authors, as can be inferred from the fact that for him Hermes was authority primarily as a teacher of astrology. So was Hermes perceived by the most of Arab authors. It is characteristic for reception of the Hermetic writings in these arab authors that they knew and interpreted mainly the writings of the so-called popular hermeticism, ie. the writings of magical and alchemical-astrological character. But most citations which Herman in De essentiis brings forth from Asclepius, the writing belonging to the learned Hermeticism, refer to the issues that were the subject of main interest for Arab students of hermetic tradition, by which is meant primarily astrology. Herman knew some other Hermetic writings (he mentions the issue from Kore kosmou not adducing the source), so he knew also one writing of early Arab Hermetica, for which Ch. Burnett argues that it significantly influenced the philosophy of Herman. This is a Liber de secretis naturae (Sirr al-haliqa or al-Khaliqua) or Book of Secrets of Creation attributed to Balinus or pseudo-Apollonius ie. Apollonius of Tyana, which in the 12th century was translated in Latin by Hugh of Santalla. In this text the elements of learned and popular hermeticism are intermingled. Based on the main theses exposed in this writing and on the comparison of them with the outline of Herman’s philosophy the article states that those texts have a lot in common. In conclusion, we answer the question “What is hermetic in Hermann’s philosophy” stating that ‘hermetic’ does not refer to a specific learning in Hermetica, but above all to the more general attitude towards the world that we find in Hermetic writings, on what Garth Fowden referred to as “hermetic mind”, which implies the attitude of an ensouled universe, the interconnection of all that is in the world, the essential relation of the microcosm and the macrocosm and so on. But these attitudes are not representative exclusively for hermetic writings. They occur within synkretistic tendencies of the Hellenistic period and during the empire within which the different traditions of ancient philosophy are merged with elements of Eastern religions. Perhaps the most accurate description of Herman’s attitudes would be the phrase of Ch. Haskins according to which Herman’s philosophy is “neoplatonism of Hermes Trismegistus”, which should be understood as neoplatonism that appeals to the authority of Hermes Trismegistus. The basis of ‘Herman’s Platonism’ is actually Arabic neoplatonism with an accentuated role of astrology, which blend elements of Plato’s, Aristotle’s and Stoic philosophy. Plato’s Timaeus is the basic text on which relies Herman when formulating the key positions of his cosmology exposed in his work De essentiis. The only source of knowledge of Plato’s dialogue Timaeus for Hermanus is the translation made by Calcidius toward the end of the 4th century. Calcidius accompanied his translation with the commentary. His translation and commentary relate only to part of the Timaeus from 17A to 53C. The article starts with the question how much does Herman, in the formulation of the fundamental positions of his philosophy and in the interpretation of Plato’s views on certain issues owes to Calcidius’ commentary. In order to answer that question in the text we espose the similarities and differences in Calcidius’ and Herman’s attitudes. Particular attention is paid to the differences, with the emphasis on the role of astrology and hermeticism in Hermann’s philosophy. These are those layers of thought and tradition, which Herman takes over and elaborates primarely under the influence of Arab philosophers. In the final part of the text we come to the conclusion that Herman knew Hermes’s writings from two sources i.e. Latin and Arabic. In the West, namely, starting from the Church Fathers, there is a continuous reception of one of the Hermetic writings, Asclepius, from which Herman delivers citations in his De essentiis. Hermes was also a great authority for the representatives of the school of Chartres, with whom Herman was closely connected. But the main source of Herman’s knowledge of the hermetic tradition are still the writings of the Arab authors, as can be inferred from the fact that for Herman Hermes is primarily the authority as a teacher of astrology. One of the main features of the reception of the Hermetic writings by Arab authors is that they know mostly the writings belonging to the so-called popular hermeticism, ie. the writings of magical, alchemical and astrological character. On the example of Herman’s reception of the hermetic tradition we have shown that the West even thanks to the Arab sources, from the 10th century on, became acquainted with the Hermetic tradition. In this final part of the text we particularly stress the importance of one of the most important works of the so-called Arab Hermetica, Liber de secretis naturae for the formating of Herman’s philosophy. Eventualy we point out that the Arab philosophers contributed to the introduction of the Hermetic tradition to the West, which corrects the current belief that this tradition is present in the West only thanks to the Latin sources. Finally, the importance of the text Liber de secretis naturae for the formation of Hermann’s philosophy is particulary emphasized. In the article we are also trying to show that thanks to those layers of Herman’s thought, which he developed under the Arabic influences, the thesis of Raymond Klibansky of the continuity between medieval and Renaissance, especially platonic philosophical thought, can be confirmed.