Hormesis is a central feature within the lives of individuals challenged by daily stresses. It is a fundamental biological concept at the core of evolutionary biology and the process of natural selection. This article shows Hormesis to be the most reliable and generalizable quantitative index of biological plasticity, constraining the magnitude of dose–response relationship as seen in the pharmacological concept of the ceiling effect and resulting from the downstream integration of activation signals from multiple pathways. It is conjectured that low doses of toxins or other stressors might activate the repair mechanisms of the body.