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 known for his early 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 two sets of four jars. In one set he placed different types of meat and fish into the different jars but left the jars open. In the other set he placed the same types of meat and fish into the jars but securely sealed the tops with paper and string. Maggots developed in the open jars but did not develop in the paper-sealed jars. The diagram below shows one pair of jars. Redi repeated this experiment and got the same results. Redi realized that some may criticize the experiment because one set of jars was open to the air and the other was sealed, potentially affecting the results. Redi followed the original set of trials with one where he placed flesh and fish into a large vessel and sealed it with fine gauze instead of paper. This would allow air to enter and leave the vessel. The maggots did not form in the vessel [_1_] .

Today controlled experiments are commonly demanded by scientific journals and are sometimes legally required by regulatory bodies (especially for pharmaceuticals).

We are taught that Galileo introduced the scientific method while Francesco Redi introduced the controlled experiment. Both beliefs are simplistic. 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 and Redi was yet to be born, 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 [_2_] .

Controlling for known variables doesn't guarantee that you will get 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. The Tower of Pisa experiment did occur even though it considered a myth (see Myth 1. The Tower of Pisa Myth). It was conducted by Vincenzio Renieri, a Catholic monk, and not by Galileo as is commonly thought. 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 different weights show that there is a tendency to grip the heavier ball more tightly and release it more slowly [_3_] .