During the time of Fransesco Redi, the 17th century, people believed in spontaneous generation of organisms from non-living thing, or abiogenesis.
The people of that time believed that living things, such as flies, arise from non-living things spontaneously. Fransesco Redi challenged this belief and set up the famous fruit fly experiment to demonstrate that life comes from pre-existing life, and not from non-living things such as meat and fruit.
