Galileo Galilei (1564-1642)

A Protagonist of the Scientific Revolution:

Galileo Galilei was one of the protagonists of the scientific revolution, best known for his astronomical discoveries by means of a telescope (including sunspots, Jupiter’s satellites, and the phases of Venus), for his defense of heliocentrism, and and for his study of the natural laws regarding falling bodies. Galileo, however, gave key contributions also to the development of the modern scientific methodology; for this reason, he holds a special place in the philosophy of science and epistemology.


Galileo was born in or nearby Pisa from Vincenzo Galilei and Giulia Ammannati. His father was a well known lutenist and music theorist, and the young Galileo was well versed in the art as well (Galileo’s youngest sibling, Michelangelo, became indeed an accomplished lutenist). During his youth, Galileo moved through different locations in Tuscany, among which the monastery of Vallombrosa, run by Camaldolese monks.

The story goes that until 1581 Galileo had not studied mathematics. After considering priesthood and enrolling into a program for physician at the University of Pisa, Galileo suddenly realized his mathematical vocation upon attending a geometry lecture. Immediately, he started discovering ingenious facts and creating crafts such as thermoscope. As early as 1589, Galileo held a chair in mathematics at the University of Pisa, which position he kept until 1592, when he moved to Padua.

There, he remained until 1610.

The two most famous episodes of Galileo’s life concern his disputes with Roman Catholic authorities. The first took place in 1619, when Galileo entered into a direct controversy with father Orazio Grassi, of the Society of Jesus. The matter at issue concerned the nature of comets.

The second dispute concerned heliocentrism and culminated in Galileo’s condemnation for heresy in 1633.

Sensed Experiences and Necessary Demonstrations:

Galileo’s success is due not only to the subtlety of his arguments, but also to the lucidity of his scientific methodology, well reflected also in the writing style. Philosophically speaking, the methodology is that part of Galileo’s work that is most relevant. He clearly demarcated between that information that is gathered through the senses (which he labeled "sensate esperienze," sensed experiences) and that which comes from deductive reasoning (labeled "necessary demonstrations").

The distinction is of great importance because it sets apart that which is understood by the intellect alone, independently of any specific experience, and that which depends on specific experiences in order to be known. Galileo was a strong defender of a form of Platonism, according to which the world is created according to certain mathematical laws, which humans can discover by properly using their rational capacities.

Subjective and Objective Properties:

Another key distinction drawn by Galileo sets apart those properties of objects that are due to the subjective characteristics of an agent and those that instead solely rest on the object’s own existence.

Thus, that a certain feather is green and ticklish does depend on the specific subject (not everyone is ticklish, after all, and not experiences colors alike); on the other hand, the shape and weight of the feather will be true of it regardless of the subjective conditions of the agent who is set to explore them.

The reaffirmation of the distinction between subjective and objective properties will play a crucial role in the development of early modern philosophy: in cognate forms, it can be found for instance in authors as different as Descartes and Locke.

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