Most people can name one 17th century Italian scientist who challenged Aristotle's writings and changed the way science was done for centuries to come. There were actually two! Galileo was one. Francesco Redi was the other. Francesco Redi is famous for his demonstration of the use of controlled experiments and his challenge to the theory of spontaneous generation.
When a scientist designs an experiment it is important to eliminate as many unknowns as possible. For instance, if one were trying to assess the health effects of a drug on humans, there are many factors which may affect health..simply counting how many of the patients get better or worse when given the drug is not good enough. We want to know how many got better or worse specifically from the drug. One solution might be to introduce a control to compare the drug-based tests against some standard case. In these drug-tests one group is commonly given the drug and another group, the control group, is given a placebo (commonly a sugar-pill with no known health effects). The subjects do not know which type of pill they have been given. The drug results from the test group can then be compared against those of the control group and we can get a better idea of which effects result from the drug. This important advance in scientific methods was introduced only 25 years after the death of Galileo and only a few kilometres away from where he lived.
Francesco Redi was able to disprove the theory that maggots could be spontaneously generated from meat using a controlled experiment. Spontaneous generation, the theory that life forms can be generated from inanimate objects, had been around since at least the time of Aristotle. Francesco took eight jars, placed meat in all the jars, but covered four of the jars with muslin. Maggots developed in the open jars but did not develop in the muslin-covered jars. Today controlled experiments are commonly demanded by scientific journals and are sometimes legally required by regulatory bodies (especially for pharmaceuticals). The image below is taken from Esperienze intorno alla generazione degl' Insetti (p. 187) where Francesco Redi published a description of the experiment in 1668 (see sidebar for digital copies of book).
We are taught that Galileo introduced the scientific method while Francesco Redi introduced the controlled experiment. Both beliefs may be simplistic, however. Francesco Redi and Galileo Galilei demonstrated their methods using very simple experiments then explained their procedures in clear and compelling ways. This is why both are so important. But scientists before Redi and Galileo had recognized the need to control variables and had described the sequence of steps described in Galileo's experimental method. When Galileo was still a young boy, Giuseppe Moletti, a professor at the University of Padua, conducted a series of experiments on free fall by dropping weights in different media (see Timeline of Classical Mechanics). His test with free fall in water and air specified that the balls must be of the same substance, weight and figure in order to remove doubt. In the same book, when Moletti described dropping balls of wood and lead from a tower to demonstrate that free fall doesn't depend on weight (as Aristotle had said) he was careful to eliminate size as a nuisance variable by conducting the experiment with wooden balls of different sizes [_1_] .
Being careful to control for the known variables doesn't guarantee that you will get the correct results. That is because "you don't know what you don't know". There might be variables that need to be controlled that you don't even know exist. This is why the famous Tower of Pisa experiment actually came up with incorrect results. Many consider the legend of the Tower of Pisa experiment to be a myth (see Myth 1, The Galileo Myths) . The experiment did occur. It was conducted by Vincenzio Renieri, a Catholic monk and another University of Pisa professor and not by Galileo as is commonly thought. Vincenzio was a friend of Galileo's. Like Moletti before him, Renieri, controlled for size when he dropped two balls of the same size (one of wood and one of lead). He came up with the wrong results. There was almost 2 metres difference between the heavier and lighter balls when they hit the ground. Galileo described similar results in some of his works. These scientists could not have known that they needed to control for human physiology as well. Modern experiments with humans dropping balls of markedly different weights show that there is a tendency to grip the heavier ball more tightly and release it more slowly [_2_] .
Francesco Redi lived during the time of the Galileo Affair. This event is presented as evidence for the "the recurring clash between religion and science" (see Galileo's Battle for the Heaven's). Francesco Redi's experiences counter this interpretation. Francesco Redi lived a comfortable life in Florence, walking the same streets and working for the same people that Galileo did (the Medicis). He died without encountering any problems with the Church. Galileo's use of Italian instead of Latin was supposed to be a problem with the Church. But with Francesco Redi, it wasn't. Any challenge to Aristotle was supposed to be a problem for the Church. It was Aristotle who proposed life-forms such as maggots spontaneously generated, and it was Redi who proved this false. The Galileo Affair was supposed to have caused the decline of science in Italy. Redi's important advances in the scientific method happened only a short time after the Galileo Affair in Galileo's adopted city.
The life and work of Francesco Redi provides cause to rethink the the Galileo Affair. The Galileo Affair is commonly presented as proof of the conflict between science and the church. Francesco Redi was defending scientific ideas that were as radical as Galileo's. His experience with the church was completely different. Could Galileo's personality and his personal and professional disagreements with the other scientists of the day explain the difference? And leaving personality aside, Francesco Redi may have had a better argument against Aristotle because he used better methods.