One of Galileo’s most often-repeated philosophical convictions is that sense experience and personal observation trump established authority in science. By observing nature, conducting experiments, and drawing conclusions, we gain new knowledge—not by merely by repeating the judgments of past thinkers. This was one of the key ideas of Galileo’s career, which led to the formulation of the scientific method as well as to the philosophical tradition of empiricism. Galileo argued this idea against philosophers and scientists who were beholden to the established authority of Aristotle and other ancient authors. To rely on outmoded authority in this way is nothing less than “to defend what is false” (“Letters on Sunspots,” 142) and thus to cause knowledge to regress. Galileo opines that Aristotle himself would want his ideas to be revised in light of new discoveries: “They go about defending the inalterability of the sky, a view which perhaps Aristotle himself would abandon in our age” (“Letters on Sunspots,” 141).

However, the findings of Aristotle, Ptolemy, and other ancients were often taken as unquestionable for other reasons besides their being considered wise thinkers. In many cases, their conception of things was also considered to accord better with sense experience than the findings of Galileo and the newer scientists. The Ptolemaic system is a good example. It seemed to accord better with our sense experience to say that the sun moved around the earth because that is what we seem to see every day. Copernicus, Galileo, and other Renaissance scientists therefore had to demonstrate that their unusual theories reflected reality; they did so by arguing that the earth must move in the same way as other planetary bodies.

The primacy of observation and experiment means that Galileo’s scientific theories too are capable of being superseded. Galileo’s awareness of this is evident in his intellectual humility and deferment to other thinkers. This is reflected in passages like the following:

That there may be more of these planets than the four hitherto observed, as Apelles says he holds for certain, may possibly be true; such positiveness on the part of a person who is (so far as I know) very well-informed makes me believe that he must have very good grounds for his assertion which I lack (“Letters on Sunspots,” 101).