It was natural philosophy. Yeah it’s pre science, but it’s far more in the realm of philosophy and while it could’ve been wrong it was a reasonable guess. They probably thought there were atoms of granite and air, but they came at it from the perspective that things can be broken down into smaller and smaller pieces, but there’s probably a smallest piece that can’t be broken. Beyond that reasonable estimate, they were basically wrong about all of it, including the “can’t be broken”