Italian philosopher and natural scientist who inaugurated the Renaissance empiricist reaction against the practice of reasoning without reference to concrete data.

Born of noble parentage, Telesio received a doctorate in 1535 and joined the group of thinkers known as the Accademia Cosentina. After spending nine years in a monastery, he lived in Naples and Cosenza. The first two books of his major work, De natura juxta propria principia (“On Nature According to Its Own Principles”), were published in 1565, and the complete edition of nine books appeared in 1586. Although Telesio had been encouraged in his writings by contemporary Roman Catholic popes, this work and two of his minor works remained on the Roman Catholic church's Index of Forbidden Books from 1596 until 1900.

The central proposition of De natura held that the only way to understand the things of the natural world was to study nature itself. This should be done, Telesio asserted, with attention to the physical properties of matter and to the aspects of heat and cold. He stated that matter is not “pure potency,” the concept ascribed to Aristotle, but rather a tangible datum, and his studies of plants and animals led him to believe that heat is the source of life, a conclusion based on the warmth that he perceived. Cold complements heat as the other active principle that explains all natural phenomena. Such a shift to evidence that is available to the senses, in place of the Aristotelian emphasis on conceptual analysis without reference to sense data, led Francis Bacon to refer to Telesio as “the first of the moderns.”

Despite his stress on the study of nature, however, and the relative lack of theological speculation in his works, Telesio also maintained a belief in God, the human soul, and immortality. Later philosophers who further developed his empirical method include the Italian thinker Tommaso Campanella and the English philosopher Thomas Hobbes.