Keep going further back. Imagine being Robert Hooke and realizing for the first time in human history that every time he looked closer at something under his microscope, there kept being tinier living things. And they didn't seem to get less complex, they kept getting smaller but were still filled with as much detail as you would see looking at a dog with the naked eye. And then to realize that the human body is absolutely full of them! Vast armies of invisible foreign living bodies inside of each person. No matter how powerful the magnification of his microscope, he never stopped discovering more tiny living things. It would have meant a complete reordering of his understanding of the world and man's place in it which most of us just accept because it was taught to us in school.