In addition to his anatomical investigations, da Vinci studied botany, geology, zoology, hydraulics, aeronautics and physics. He sketched his observations on loose sheets of papers and pads that he tucked inside his belt. He placed the papers in notebooks and arranged them around four broad themes—painting, architecture, mechanics and human anatomy. He filled dozens of notebooks with finely drawn illustrations and scientific observations. His ideas were mainly theoretical explanations, laid out in exacting detail, but they were rarely experimental.