Rationality and truth

by Tim Harding

Rationality is the state or quality of being rational, which means being consistent with or based on or using reason.  Reason is thought by rationalists and skeptics to be more reliable in determining what is true; in contrast to reliance on factors such as authoritytradition, instinct, intuitionemotionmysticismsuperstitionfaith or arbitrary choice.

Harvard philosophy professor Robert Nozick has proposed two criteria for rational belief:

  1. support by reasons that make the belief credible (e.g. scientific evidence); and
  2. generation by a process that reliably produces true beliefs (e.g. the scientific method).[1]

For instance, until early December 2010, science told us that the element arsenic is toxic to all life on Earth, in even very small concentrations.  But then NASA announced that scientists had discovered a microorganism in California’s Mono Lake able to thrive and reproduce using arsenic instead of phosphorus in its biochemistry.[2] In terms of Nozick’s criteria, it was rational until December 2010 to believe that arsenic is toxic to all life on Earth, even though we now know that the belief was false.  Was it rational to hold this belief after the NASA announcement?  Using the same criteria, our answer would be ‘no’.

A statement is true when it represents how things are; true statements are ones that correctly describe reality; true statements correspond to the way the world really is. But as we have seen, a rational belief is not necessarily true.  Conversely, an irrational belief is not necessarily false.  For example, a prediction made by a psychic can turn out to be true by coincidence.  On the other hand, a rational belief needs to be reasonable or credible in the circumstances; that is, a rational belief is one that is justified by reason.

What we can say that is because an irrational belief is unreliable and more likely to be false than a rational belief, we should therefore be more skeptical about beliefs that are known to be or appear to be irrational than about rational beliefs.


[1] Nozick, R. (1993) The Nature of Rationality, Princeton University Press, Princeton.

[2] http://www.nasa.gov/topics/universe/features/astrobiology_toxic_chemical.html

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