Spontaneous generation is an outdated scientific theory that suggested living organisms could arise from non-living matter. For example, it was 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 eventually disproven through experiments by scientists like Louis Pasteur and Francesco Redi. They demonstrated that life comes from existing life, not from inanimate objects. This led to a better understanding of biogenesis, the principle that living organisms originate from other living organisms.