Spontaneous generation is the outdated scientific theory that living organisms can arise from non-living matter. For example, it was once believed that maggots could spontaneously appear from decaying meat or that mice could emerge from old rags. This idea was widely accepted until the 17th century. The theory was challenged by experiments conducted by scientists like Louis Pasteur and Francesco Redi. Their work demonstrated that life comes from existing life, leading to the development of the modern understanding of biogenesis. This shift in thinking laid the groundwork for advancements in microbiology and the study of cellular life.