Today, Italian scientist Galileo Galilei is mainly remembered for his perfection of the telescope and discovery of the four largest moons of Jupiter. However, in his astronomical observations, Galileo also made an important contribution to the development of deductive reasoning during the 17th century. Debates about inductive and deductive reasoning among philosopher-scientists of that century led to the emergence of the scientific method.
According to philosopher Lee C. Archie of Lander University, there are basically two types of formally correct reasoning. Deductive reasoning begins with a set of general premises and uses them to derive truth about a specific case. For instance, Archie writes, we could begin with the observation that all animals have DNA, and that a fruit fly is a form of animal. From those premises, we could conclude that one particular fruit fly, which we are examining, has DNA. Inductive reasoning works the opposite direction, beginning with a set of specific observations and attempting to derive general truths. At the time Galileo was conducting its research, philosophers debated which approach was best for studying the natural world. The scientific method ultimately combined both methods by requiring rigorous testing of all hypotheses.
The deductive reasoning used in Galileo's astronomical observations is the subject of a paper in the journal Science & Education by Anton E. Lawson, who is a professor of organismal, integrative and systems biology at Arizona State University. Lawson writes that "unlike many modern scientific papers," Galileo's first report of his astronomical observations in 1610 "is striking in the way in which it chronologically reveals many of the steps in his discovery process." What he means is that, whereas the description of the research methods is a brief and separate part of scientific papers today, Galileo presented his findings in the form of a narrative, or story, detailing what he did, what he observed as he did it, and even his emotions as he successfully operated his newly built telescope. That, says Lawson, means we get unique insights into the scientific mind of Galileo.
With that in mind, Lawson proceeded to explore how Galileo would have formulated his ideas about planets and moons. Galileo would probably have believed that there were three types of objects in the sky: fixed stars, which moved only with the rest of the "firmament" (that is, the sky); planets, which orbited the Sun; and moons, which orbited the planets. (At the time, only Earth's Moon was readily visible with the naked eye.) Then, Lawson says, as he identified new objects through his telescope, he would evaluate what they were by comparing them to what he thought he knew about celestial objects. Over the course of several nights of observations, Galileo confirmed that several unusually bright spots he spotted in his telescope could not be fixed stars, because they moved, nor could they be planets, because they moved from side to side of an already known planet, Jupiter. They must therefore be moons, like Earth's, orbiting the planet of Jupiter.
Although Galileo certainly did make important astronomical observations, not all scientists are convinced that he was using the sort of deductive reasoning described by Lawson. For instance, University of Minnesota philosopher of science Douglas Allchin says that Lawson is "shoehorning history into a particular view of scientific methodology."