This article explores some connections between the medical conception of poison-induced epilepsy and the ontological conception of a plurality of substantial forms in living beings in the work of the Wittenberg physician and philosopher Daniel Sennert (1572–1637). It does so by taking a developmental approach, tracing Sennert’s responses to some of his predecessors such as Jean Fernel, Petrus Severinus, and Julius Caesar Scaliger. Sennert’s responses to Fernel indicate that Sennert does not regard poison-induced epilepsy as a disease that affects the dominant form of a living being. His responses to Severinus indicate that he also does not reduce the agency of epilepsy-inducing poisons to chemical causation. His responses to Scaliger indicate that he assigns to subordinate forms in the human body a central role in explaining the occurrence of auto-generated poisons leading to epileptic fits. At the same time, Sennert substantially goes beyond Scaliger by applying some of Severinus’s insights concerning analogies between species degeneration and the generation of disease to the case of epilepsy.