Before the 1700s some people believed that life came from mud or decaying meat what is this theory called

This theory is called spontaneous generation: before the 1700 people believed that life could be generated spontaneously from mud or decaying meat, because they observed that, after some time, flies appeared from decay meat, leading them to the conclusion that life could be spontaneously generated.

This theory was refused from both Louis Pasteur and Francesco Redi: they both demonstrated, with similar experiments, that the spontaneous generation theory was false.

The experiment is quite simple, and it’s based on two pieces of meat: one is left exposed to the air, one is covered to prevent exposure. After some time, from the piece of meat exposed some flies started to appear, while the covered piece of meat was free of flies.

This result proved that the theory of spontaneous generation was false.

The experiment is based on a principle called "manipulated variable": a single variable is the object of a controlled experiment, in which two test is conducted at the same time, in order to prove that the difference of results in the two tests is caused by the only different variable (in this case, the variable is the exposure to the air and the hypothesis is that the flies appeared in the decaying meat not because of spontaneous generation but because larvas where carrying out by air).