One of the most important concepts in toxicology was espoused in the sixteenth century by a scientist by the name of Paracelsus. He was born Philippus Theophrastus Aureolus Bombastus von Hohenheim near Zurich in 1493 and was the son of a physician who was interested in chemistry and biology and was an expert in occupational medicine. Paracelsus was a free thinker who disagreed with the dogma current at the time and espoused by Galen. Paracelsus thought observation was crucial and understood the importance of chemistry in medicine. He believed like cures like contrary to Galen who taught that diseases of a particular intensity would be cured by a medicine of opposite intensity. Consequently in the view of Paracelsus a poison in the body would be cured by a similar poison but the dosage is very important.
He advocated inorganic chemicals, such as salts, as treatments. These were believed to be too poisonous but he emphasized that the dose was very important. Paracelsus summarized this concept in the following famous phrase: A!! Substances are poisons; there is none that is not a poison. The right dose differentiates a poison from a remedy.
This concept is especially crucial to the safe use of drugs but also important for the safe handling of other chemicals (see below this chapter). It underlies the risk assessment of chemicals because from this relationship follows assessment of threshold doses and safe and non-toxic levels (see below Chapter 12). Even seemingly innocuous substances such as common salt could become poisonous under certain conditions. Paracelsus also believed that diseases were focalized to particular organs and also that poisons would damage particular organs (target organs) something we now also known to be generally true.