"Spontaneous Generation" is a theory first synthesized by Aristotle that explained the origins of life - arguing that life would simply appear given the right combination of (inanimate, non-living) ingredients.
While we find this laughable now, it's worth bearing in mind that this was virtually unquestioned for almost 2000 years (or only 1000 years, if you bought the New Chronology ...). And, in it's own way, it made sense. If you piled rags and oatmeal in the corner of your hut they would spontaneously produce mice. A lump of meat left out for a few days would of it's own accord create flies. And, like a good recipe, if you varied the ingredients you'd get different results. Extra oatmeal might mean more mice, keeping the meat wet would ensure the flies and maggots appeared.
So it was, in it's day, as common sensical as the notion of a heliocentric or geocentric universe.
Consider a few of the illusions we cherish as "Common Sense", and how curious they'll undoubtedly appear to future generations.