It is the attempt to reduce ideas or other mental elements to basic simple/ elementary ideas (sometimes just simple, unstructured sensations) that are associated together in the mind, usually through experience. Simple, often additive rules are thought to predict the properties of complex ideas from the properties of their constituent simple ideas. Associationism exemplifies the bottom-up approach.
Associationism is an attempt to reconstruct the human mind from sensory experience with minimal theoretical assumptions (Anderson and Bower, Human Associative Memory, John Wiley, 1973).
The origins of associationism can be traced to Aristotle, who formulated four laws of association of things or events in recall: law of contiguity (in space or time), law of similarity, law of contrast and law of frequency (often-ness of linking).
Associationism in various forms was accepted by some rationalists (eg. Hobbes) but it is more compatible with empiricism (eg. John Locke (1632-1704), David Hartley (1705-1757), David Hume (1711-1776), James Mill (1773-1836), John Stuart Mill (1806-1873) ...).
Associationism in the 19th C gained plausibility from the atomistic approach of chemistry and physics: explaining complex wholes as groups of units, elements or atoms connected in specifiable ways. Work in this tradition contributed to the emergence of the discipline of psychology independent of metaphysical speculation.
Although associationism lost ground in the early (Classical AI) phase of cognitive science (it cannot account for cognitive phenomena like recursive grammars), it remained alive in traditional psychology and is now seen in modified form in the connectionist AI approach.
Alexander Bain (1818-1903), an advocate of the British school of Empiricism (also worked on English language and logic), proposed (The Senses and the Intellect, 1855; Emotions and the Will, 1859) that knowledge and mental processes involve not just spontaneous thought and ideas; they are based on actual physical sensations. He catalogued correlations of physiological and psychological processes. In 1876, Bain founded the first psychological journal, Mind.
Bain studied what later came to be known as the ``Law of Effect'' (the relation of movement with feeling). Example: during a moment of physical pain, if there occurs an accidental, spontaneous movement which relieves the sensation of the pain, the organism will naturally sustain this movement as long as the painful stimulus remained. Repetition of the event would strengthen this ``association''. Thus responses to pleasure/pain stimuli could be learnt by trial and error - led later to behaviourism (which however rejected pleasure/pain states) as well as to functionalist studies of adaptive behaviour.) Bain held that beliefs are behavioural dispositions.