The notion of spontaneous generation defends the idea that Life can appear spontaneously from non-living matter.
We now know that only the living can engender the living, but the principles of spontaneous generation lasted nearly two millennia. The church defended this thesis, but great scholars such as Democritus, Aristotle and Descartes are also associated with it. According to them, the appearance of mold and insects on decomposing foods is a testament to this principle.
Francesco Redi published in Florence in 1668 his treatise Esperienze intorno alla generazione degl'insetti ("Experiments on the Generation of Insects") in which he contradicted, with the support of experiments, this theory. He responded to all his critics with great caution because he could not ignore what Galileo had suffered 50 years earlier (who was accused of heresy in 1633).
Despite his discoveries, scientists and the public continue to believe in spontaneous generation. It was not until the work of Pasteur in the mid-19th century that this theory was refuted and the existence of germs and microorganisms were validated.