Foundations Of Cognitive Science

Behavioural Indeterminancy

Behavioural indeterminancy is the claim that in principle psychology is restricted to establishing weak equivalence. Weak equivalence is equivalence with respect to input/output behaviour -- that is, two systems that generate the same kinds of responses, but for different reasons.. Therefore, measuring behavioural data is unable to establish equivalence at the underlying levels of the algorithm and of the functional architecture. Behavioural studies are indeterminate with respect to strong equivalence.

This issue is of importance to cognitive psychology because, if true, it implies that cognitive psychology cannot generate insight into cognition without importing knowledge based on non-behavioural observations from other disciplines. A famous example of this is provided by Anderson (1978), who argued that experimental evidence could not be used to choose the propositional camp vs the depictive camp in the debate about the underlying nature of mental images.


  1. Anderson, J. R. (1978). Arguments concerning representations for mental imagery. Psychological Review, 85, 249-277.
  2. Pylyshyn, Z. W. (1989). Computing in cognitive science. In M. I. Posner (Ed.), Foundations of cognitive science, Cambridge MA: MIT Press.

(Revised December 2009)