1. Some explanation of appearance of maggots or some microorganisms on rotten food was the spontaneous generation, meaning there is no parent organism, so these organisms appeared spontaneously.
Redi explained that maggots are from flies, and Spallanzani showed that rotten food is the result of growing populations of microorganisms. Pasteur explained that organisms can live without air. So, their experiments established that there is no spontaneous generation, that maggots are from flies and that microorganisms can live in the air.
2. Therefore, life must come from life.
3. They extrapolated their experiments on other situations.