/u/elprophet is right. Aristotle wrote of spontaneous generation and even mentions rotting vegetables.

"Now there is one property that animals are found to have in common with plants. For some plants are generated from the seed of plants, whilst other plants are self-generated through the formation of some elemental principle similar to a seed; and of these latter plants some derive their nutriment from the ground, whilst others grow inside other plants, as is mentioned, by the way, in my treatise on Botany. So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs."

-History of Animals, Book V, Part I by Aristotle. (https://ebooks.adelaide.edu.au/a/aristotle/history/)

It was disproved by Louis Pasteur in 1862. He boiled some broth and had one batch open to the air and allowed dust to settle on it. Another batch had a swan neck duct on it, such that air could reach the broth, but dust could not. The broth that was open to the air spoiled, whilst the other did not. This was evidence required to disprove Aristotle's theory.

edit:

"Animals and plants come into being in earth and in liquid because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full of soul. Therefore living things form quickly whenever this air and vital heat are enclosed in anything. When they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble."

-On the Generation of Animals, Book III, Part 11

Another quote about the original theory

Related- how did people explain that food or bodies decomposed slower in colder temperatures?