The Argument of Two Italians Over Science and the Senses

The prestige of the scientific revolution goes largely unquestioned. The power of its discoveries, the usefulness of its results, the speed of its progress all win approval when compared to the ancient worldview that preceded it. But despite modern characterizations, the ancient view—especially as articulated by Aristotle—was and remains broader than its modern counterpart. Modern science may be more productive and accurate, yet it eschews the philosophical resources to ground and measure its significant achievements. Two titans of the Western canon, Galileo and Thomas Aquinas, help illustrate the point.

Galileo is best known for the groundbreaking discoveries he achieved with his telescope. Through what was then a remarkably new observational instrument, he helped to topple the long-held geocentric view of the universe in favor of the heliocentric alternative. The former view was largely understood in terms of Aristotle’s four causes: material, formal, efficient, and final. Materially, the celestial bodies were characterized as being composed of a different stuff, which accounted for their distinct movement; the efficient cause of their movement was their attraction to the outermost sphere, that of the unmoved mover; their formal cause was their movement in perfect circles befitting eternal bodies; their final cause was their relation to the unmoved mover.

The intellectual revolution that Galileo contributed to changed all this. It held celestial bodies were made of matter, just like the earth; the efficient cause of their movement was neither an outer sphere nor the earth, but rather the sun, which has no privileged location; the form of their movement was governed by the same laws that govern all movements, even of earthly things; and the final cause of celestial movement, as with modern science generally, was considerably less central or altogether ignored. 

There are some key assumptions behind Galileo’s philosophy. In his essay The Assayer, published 400 years ago on October 1623, Galileo lays out the details of his epistemology. He focuses on the senses, the first element of our cognition, almost exclusively through material and efficient causes. Invoking one of the basic claims of atomism—an ancient philosophy dating from the fifth and fourth centuries BCE—he regards all material entities to be collections of tiny particles, which he calls “corpuscles.” Galileo then claims that our sensations are explained not by objects, but by the qualities of the object’s corpuscles. Something might taste pleasant or harsh, for example, because of the shape of the corpuscles as they contact the tongue (smooth or jagged respectively). Next, he develops the distinction between primary and secondary qualities. He deemed that primary qualities (ex: size, shape, and motion) truly belong to corpuscles, and by their conglomeration, the objects they compose. By contrast, we impute secondary qualities, which we derive from our senses, onto objects because of their effect on our sensory organs.