In the treatment of disease, Celsus' principal method was to observe and watch over the operations of Nature, and to regulate rather than oppose them, conceiving that fever consisted essentially in an effort of the body to throw off some morbid cause, and that, if not unduly interfered with, the process would terminate in a state of health. On occasions, however, he boldly recommends the use of the scalpel; his rules for blood letting and purgatives are laid down with detail and precision; and many of the rules he prescribes were not very different from those still in use at the beginning of the 19th century.