Daniel Kahneman Explains The Machinery of Thought
System 1 operates automatically and quickly, with little or no effort and no sense of voluntary control.
System 2 allocates attention to the effortful mental activities that demand it, including complex computations. The operations of System 2 are often associated with the subjective experience of agency, choice, and concentration.
Over at Farnam Street, an excellent introduction to Kahneman’s work.