[1] A critical philosophical question concerns the relationship between constant conjunction and causation, which has implications in the philosophy of science.
The philosopher David Hume used the phrase frequently in his discussion of the limits of empiricism to explain our ideas of causation and inference.
British empiricism and associationist philosophers elaborated on Hume's fundamental idea in many diverse ways, and metaphysicians like Immanuel Kant tried to dissipate the position.
The force of Hume's arguments has remained remarkably robust, and they have found unexpected support in three scientific discoveries of the 20th century: Ivan Pavlov's laws of conditioning; Hebbian neural networks; and spike-timing-dependent plasticity (STDP).
Modern neuroscience has confirmed this insight as a product of the activity of synapses and STDP, so structured to strengthen connections between cells that fire within very short periods (tens of milliseconds) of each other.