This thesis focuses on the scientific and philosophical though of Giovan Battista Della Porta (1535-1615). In particular, the purpose of my thesis is to analyse and “reconstruct” the notion of “secret of nature” in Della Porta’s philosophy and in his project for the reformation of natural magic.As I have shown here, Della Porta’s rationalization of the “secret of nature” is connected either with demonological issues and with astrological problems.The thesis is divided into four parts. In the first part, the historiography and reception of the philosophical thought of Della Porta are discussed. The accent is put on the meaning of natural magic with particular regard to the role he assigns to demons. Here I show that the issue is still debated in the recent historiography. The second part introduces the relationship between Della Porta’s thought and the neapolitan aristotelianism. I proposed, here, to understand the notion of “secret of nature” in light of the debates related to the knowledge of “singular”. I have shown that the matter of the statute of natural magic is related to the discussions on the medical epistemology and on the Avicennean conception of forma speciei. The third part deals with the matter of causality and the debate about astrology. Also, it analyses the meaning of the concept of “sympathy”, interpreted as celestial influence. I have shown that Della Porta reinterprets the concept of “sympathy” of Ficino as well as the notion of “astrological image” of Albert the Great. In the fourth part, I studied the relationship between the concepts of “friendship” and “sympathy” in the astrological debate. Then, I studied the principle of “similarity” in Della Porta’s phisiognomy books and in his research methodology about “secrets of nature”. I have shown that this research methodology is based on a concept of causality not concerning the substance, but the “accidents”, i.e. the “individual particularities”: the movement, the color, the figure, etc. According to Della Porta, the knowledge of the singular is based on these “individual particularities”.