After a brief overview of the contributions of Paracelsus, Haber and Arndt to the theory of toxicology, examples are provided for quantitative risk/safety assessments using dose (c), time (t) and effect (E) as macroscopic variables of toxicity. The discussion offers explanations for application of the decision tree approach in identifying rate-determining steps in the toxicity of chemicals. Having done so allows for reasonably accurate predictions of cancer incidence (bladder, liver, heart, histiocyte) using Haber's Product under isoeffective conditions and the equation cxt=kxE for isodosic and isotemporal responses.