The article is the third serial fragment of author's doctor's thesis entitled 'Witold Rubczynski as a Philosopher and Historiographer of the Philosophy' (for previous parts see 'Quarterly Journal of the History of Science and Technology' no. 3-4(20060 and no. 1(2007)). In the paper he is presenting works of Witold Rubczynski that concern European medieval philosophy, its range, method, findings, and their immediate interest. As a pioneer of the Polish philosophical medieval studies Rubczynski takes up his research works in the latter part of the 19th century. Since then European medieval studies started to develop. The main matter of his interests was Neo-Platonism, and its reception in the Middle Ages. The author of Neo-Platonic Studies included in his searches attainable literature concerning the trend, and manuscriptal sources as, for example, treatise of the 13th century entitled 'De intelligentiis', and two commentaries on 'Elementatio Theologica' of Proclos, Bertold of Moosburgh (13th c.) and Wilhelm Moerbecke (14th c.). The method that was mainly based on philosophical analysis of the manuscripts' contents, and which was used by Rubczynski, sometimes turned out to be deceptive. However, getting more precise results was not possible as far as the searches' results of that time are concerned. Today we realize that it was necessary to engage a large number of researchers in making an effort to obtain an appropriate solution. The scientific research of Rubczynski overthrew the common among medievalists opinion telling that philosophy in the 13th and 14th century was strongly dominated by Aristotelism. The searches also showed that Neo-Platonism was very vivid all the Middle Ages. Moreover, the trend had not only influenced scholastic philosophy, but also cleared the ground for modern philosophy. Such image of the epoch was confirmed by ensuing researchers, whose works were published in the 1960s.