System 1 and  System 2 refer to the psychological dual-process theories developed in  cognitive psychology which contrast analytical reasoning and non-analytical reasoning as two modes of knowing and thinking processes.(1;2) The system 1 or the non-analytical system (see also in this glossary) can be seen as a process leading rapidly to the selection of the preferred management options for the target condition.(3) The system 1 is implicit, based on automatic and effortless thought processes and is associative, intuitive and fast. Non-analytical, intuitive thinking is explained in terms of the high accessibility of the immediate thoughts(4). Non-analytical reasoning can be recognized both in medical decision-making and in medical problem-solving, for instance in automatic chance assessment processes and in pattern recognition(5). It acts unconsciously and is used most of the time by the health care experts for common conditions.(6)

The system 2 or the analytical system (see also in this glossary) is explicit, controlled, rational, effortful and relatively slow. In clinical reasoning, analytical thinking is present in deliberately generating and testing of diagnostic hypotheses, in causal reasoning with biomedical knowledge, and in the use of decision tools. It can locate information which enables a decision to be made when System 1 is incapable of doing so. Moving into a System 2 process requires a conscious and deliberate effort(7).

The interaction between these two systems is considered to determine the output of the whole thought process. The outcomes of the system 1 (non-analytical system) can be reflected upon by the system 2 (analytical system) and accepted or elaborated upon for further understanding and investigation or to provide explanations.(7)

(1) Croskerry P. A universal model of diagnostic reasoning. Acad Med 2009 Aug;84(8):1022-8.

(2) Evans JS. In two minds: dual-process accounts of reasoning. Trends Cogn Sci 2003 Oct;7(10):454-9.

(3) Kahneman D. A perspective on judgment and choice: mapping bounded rationality. Am Psychol 2003 Sep;58(9):697-720.

(4) Kahneman D, Frederick S. A Model of Heuristic Judgement. In: Holyoak KJ, Morrison R, editors. The Cambridge Handbook of Thinking and Reasoning.New York: Cambridge University Press; 2005. p. 267-93.

(5) Epstein S. Integration of the cognitive and the psychodynamic unconscious. Am Psychol 1994 Aug;49(8):709-24.

(6) Boreham NC. The dangerous practice of thinking. Med Educ 1994 May;28(3):172-9.

(7) National Prescribing Centre (NPC) provided by NICE for the NHS. Making decisions better. MeReC 2011;22:1-8.