Spontaneous generation is an obsolete scientific theory that proposed living organisms could arise directly and regularly from non-living matter without the need for parent organisms or seeds. For example, it was believed that maggots could spontaneously appear from decaying meat, fleas could arise from dust, and mice could emerge from a mixture of bread and cheese left in a dark corner

. This idea was first coherently formulated by Aristotle around 350 BC, who suggested that life could emerge from the combination of earth, water, air, and a "vital heat" or "soul" present in these elements. Aristotle believed that this vital heat caused non-living matter to generate living creatures under suitable conditions

. The theory of spontaneous generation was widely accepted for about two millennia until it was challenged and eventually disproved in the 17th to 19th centuries by experiments from scientists such as Francesco Redi, Lazzaro Spallanzani, and most decisively Louis Pasteur. Pasteur’s work showed that microorganisms do not arise spontaneously but come from other microorganisms, leading to the modern understanding that life arises from pre-existing life (biogenesis), not spontaneously from non-life

. In summary, spontaneous generation is the historical belief that life can arise from non-living matter directly and repeatedly, a theory now rejected and replaced by modern biology and the concept of abiogenesis, which addresses the origin of life on Earth over millions of years but not ongoing spontaneous generation today