Spontaneous generation is the idea that life can just pop into existence. Today, this sounds silly. It sounds like some Hand of God idea that the almighty looks down one day, thinks, that field could look a little prettier with just one more sheep in it and poof, he's made a new sheep.

But this is not about making sheep.

No, actually spontaneous generation is about conservation of matter. Matter in the form of living things doesn't just appear, existing matter from other things unites to build a new living thing. This is like anti-entropy (and probably does need some sort of divine inspiration to input the driving energy). The universe has an internal sense of order where life, "sweet, screaming, pooping life", is a higher order state. It was Aristotle's baby. Anaximander, Hippolytus and Anaxagoras were also pretty keen on it, and thought life would emerge from slime, mud and earth, so long as there was sunlight on it. This is actually a pretty cool and counter logical deduction. Then Pasteur had to go and spoil it by showing that meat in a sealed container didn't develop maggots.

This put a stop to the theory because if spontaneous generation had been hand of god derived, it wouldn't make much sense to say god could not get into your sealed box, i.e. he just can't think inside the box. But if the matter that is supposedly conserved is locked into the box, and no life sprouts, and in that which is not boxed up, life does sprout (and you repeat it many times until your statistics are convincing), you eventually have to come to the conclusion that life comes from outside the meat, not through conservation of the matter in the meat, and then all you need to do is develop a good enough microscope to see the fly eggs.