Rudolf Steiner (1861-1925) was an Austrian architect, philosopher, initiate, esotericist, and clairvoyant. He is considered one of the greatest and most influential spiritual masters of the twentieth century. Theosophy: An Introduction to the Supersensible Knowledge of the World and the Destination of Man, published in 1904, is one of Rudolf Steiner's foundational texts. In this essay, Steiner does not merely present the principles of Theosophy as he had learned them, but he also lays the groundwork for what would become his own Anthroposophy. The text is structured into four main parts, in which Steiner presents his vision of the human being and the cosmos. In summary, Theosophy is a crucial work for anyone who wants to understand Steiner's thought. It is not just a treatise on Theosophy, but a manifesto of his spiritual science, which seeks to unite the scientific knowledge of the physical world with the spiritual knowledge of the supersensible world. The work is a bridge between the traditional esoteric view and a more systematic and rational approach that would characterize Anthroposophy throughout its subsequent evolution.