The term ‘stimulus’ traverses the depth-psychology corpus along several intersecting axes, none of which can be reduced to a simple input-output schema. At the neurobiological pole, Kandel’s work on Aplysia treats the stimulus as the irreducible experimental unit through which habituation, sensitization, and classical conditioning are operationalized — a weak or strong electrical pulse that either strengthens or attenuates synaptic efficacy and, by extension, memory. Kornorski’s distinction between excitability and plasticity, as Kandel reconstructs it, establishes the stimulus as the precipitating event whose consequences differ depending on whether they are transient or structurally enduring. At the psychophysiological pole, Jung’s association experiments deploy the stimulus-word as a probe that selectively disturbs reaction times, galvanic responses, and reproductions wherever an emotionally charged complex is constellated — making the stimulus an instrument of diagnostic depth. Damasio and LeDoux introduce a regulatory-constructivist tension: the stimulus is neither a raw signal nor a fully determined cue but a perception requiring cortical appraisal before its motivational significance is registered. Barrett pushes furthest from the reflex model, arguing that the stimulus-response view of the brain as primarily reactive is simply wrong. Inwood’s Stoic scholarship adds a philosophical stratum, tracing how Chrysippus situated the external stimulus at the origin of a causal chain leading through assent and impulse to action. Taken together, the corpus reveals ‘stimulus’ as a site where neuroscience, experimental psychology, philosophy of action, and phenomenology meet — and productively contest one another.