Many consider Paracelsus (1493–1541) as the father of modern toxicology.

• He was the first to emphasize the chemical nature of toxic agents.
• He stressed the need for proper observation and experimentation regarding the true response to chemicals.
• He underscored the need to differentiate between the therapeutic and toxic properties of chemicals when he stated in his Third Defense, "What is there that is not poison? All things are poison and nothing [is] without poison. Solely, the dose determines that a thing is not a poison."

The introduction of the dose–response concept might have been his most important contribution to toxicology, meaning that everything is toxic at the right dose (even oxygen and water).