Constant conjunction

Inphilosophy,constant conjunctionis a relationship between two events, where one event is invariably followed by the other: if the occurrence of A is always followed by B, A and B are said to beconstantly conjoined.A critical philosophical question concerns the relationship between constant conjunction andcausation,which has implications in thephilosophy of science.

Overview

The philosopher David Hume used the phrase frequently in his discussion of the limits ofempiricismto explain our ideas of causation andinference.InAn Enquiry concerning Human UnderstandingandA Treatise of Human Nature,Hume proposed that the origin of our knowledge of necessary connections arises out of observation of theconstant conjunctionof certain impressions across many instances, so that causation is merely constant conjunction—after observing the constant conjunction between two events A and B for a duration of time, we become convinced that A causes B. However, this position raises problems, as it seems that certain kinds of constant conjunction are merely accidental and cannot be equated with causation. For example, we might observe sunrise following the crowing of rooster for a long period of time, but it would still be irrational to then believe the crowing causes the sunrise. Along these lines, a more modern conception would argue thatscientific lawis distinguishable from a principle that arises merely accidentally because of the constant conjunction of one thing and another, but there is considerable controversy over what this distinguishing feature might be.

British empiricismandassociationistphilosophers elaborated on Hume's fundamental idea in many diverse ways, and metaphysicians likeImmanuel Kanttried to dissipate the position.Kant was motivated to develop his philosophy by Hume's argument, which he considered to be an attack on science.

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;Hebbianneural networks;and spike-timing-dependent plasticity (STDP).

In Pavlov's framework, anunconditioned stimuluscan follow in constant conjunction a conditioning/conditioned stimulus within a timeframe of milliseconds to several seconds, and result in theconditioned stimulushaving many of the properties of the unconditioned stimulus.Donald Hebbexplained this as an intrinsic property ofcell assemblieswithin the nervous system to form connections within large cliques of cells whenever those cells fire together within a reasonably short period of time. (A modern shorthand for his ideas states: "Cells that fire together, wire together".) Modernneurosciencehas 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. The longer time periods of classical conditioning are presumably a large-number effect of cliques of these synapses and cells.

External links

• "David Hume: Causationat theInternet Encyclopedia of Philosophy– A discussion of alternative interpretations of Hume's position
• "Kant and Hume on Causality"at theStanford Encyclopedia of Philosophy