Petrus Peregrinus of Maricourt, a 13th-century French scholar and engineer, wrote what we can consider as the first extant treatise on magnetism of Europe. This treatise is in the form of a letter, probably composed during the siege of Lucera in Italy, in 1269, where Peregrinus worked to fortify the camp and built engines for projecting stones and fireballs into the besieged town. Peregrinus' letter consists of two parts. The first is discussing the properties of magnets, describing also the methods for determining their north and south poles. The second part of the letter describes some instruments that utilize the properties of magnets, ending with the Peregrinus' art of making a wheel of perpetual motion. In this paper, we discuss the first part of the letter and the related medieval knowledge of magnetism.