He was educated at Milan by his uncle, Antonio, himself a scholar and a poet of eminence, and afterwards at Rome and Padua. His studies included all the wide range of subjects, classics, science and philosophy, which constituted the curriculum of the Renaissance savants. Thus equipped, he began his attack upon the medieval Aristotelianism which then flourished in Padua and Bologna. Resigning to his brother the archbishopric of Cosenza, offered to him by Pope Pius IV, he began to lecture at Naples and finally founded the academy of Cosenza. In 1563, or perhaps two years later, appeared his great work De Rerum Natura, which was followed by a large number of scientific and philosophical works of subsidiary importance. The heterodox views which he maintained aroused the anger of the Church on behalf of its cherished Aristotelianism, and a short time after his death his books were placed on the Index,
Telesio was the head of the great South Italian movement which protested against the accepted authority of abstract reason, and sowed the seeds from which sprang the scientific methods of Campanella and Bruno, of Bacon and Descartes, with their widely divergent results. He, therefore, abandoned the purely intellectual sphere and proposed an inquiry into the data given by the senses, from which he held that all true knowledge really comes. Instead of postulating matter and form, he bases existence on matter and force. This force has two opposing elements: heat, which expands, and cold, which contracts. These two processes account for all the diverse forms and types of existence, while the mass on which the force operates remains the same. The harmony of the whole consists in this, that each separate thing develops in and for itself in accordance with its own nature while at the same time its motion benefits the rest. The obvious defects of this theory, (i) that the senses alone cannot apprehend matter itself, (2) that it is not clear how the multiplicity of phenomena could result from these two forces, and (3) that he adduced no evidence to substantiate the existence of these two forces, were pointed out at the time by his pupil, Patrizzi.
Moreover his theory of the cold earth at rest and the hot sun in motion was doomed to disproof at the hands of Copernicus. At the same time, the theory was sufficiently coherent to make a great impression on Italian thought. When Telesio went on to explain the relation of mind and matter, he was still more heterodox. Material forces are, by hypothesis, capable of feeling; matter also must have been from the first endowed with consciousness. For consciousness exists, and could not have been developed out of nothing. Again, the soul is influenced by material conditions; consequently the soul must have a material existence. He further held that all knowledge is sensation ("non ratione sed sensu") and that intelligence is, therefore, an agglomeration of isolated data, given by the senses. He does not, however, succeed in explaining how the senses alone can perceive difference and identity. At the end of his scheme, probably in deference to theological preiudices, he added an element which was utterly alien, namely, a higher impulse, a soul superimposed by God, in virtue of which we strive beyond the world of sense.
The whole system of Telesio shows lacunae in argument, and ignorance of essential facts, but at the same time it is a forerunner of all subsequent empiricism, scientific and philosophical, and marks clearly the period of transition from authority and reason to experiment and individual responsibility. Beside the De Rerum Natura, he wrote De Somno, De his guae in acre fiunt, De Mari, De Comelis et Circulo Lactea, De usu respirationis, etc.