The conclusions from behavioral research and brain-imaging research are supported by observations of patients with injuries to certain parts of the brain. People with left parietal injuries can lose the ability to understand quantities of things, but keep at least some ability to do exact arithmetic, such as multiplication. People with left-hemisphere brain damage can lose the ability to do exact arithmetic, but keep a sense of quantity, including the ability to compare larger and smaller numbers. This information confirms that distinct parts of the brain are used to know and use approximate and exact arithmetic.