This paper examines Columbus' first trans-Atlantic voyage by studying Diario, the only remaining material among the four voyages. It analyses the contents of the books that he owned and read meticulously, making hundreds of notes on the margins. Diario will help reconstruct not only his navigation process across the Ocean and his contact with the inhabitants of the Americas, but also his view of the world and mentality in general. Certainly he gained his geographical knowledge and view of the globe from many classical works. One of the most important books that contributed to the formation of his 'medieval' mental structure was Imago Mundi of Pierre d'Ailly. It provided him with the basis of his reasoning about the form and scale of the globe. He tended to underestimate the size of the earth itself while overestimating the size of the Eurasian continent and this led to his optimistic view of trans-Atlantic navigation. After several months' of encounter with the indigenous people of the newly found continent, his attitudes transformed gradually into the direction of domination and exploitation. The dynamics of the so-called Discoveries was drawn from the traditional mentalities of the Old Continent of Europe.