As shown in Figure 2, the model assumes that critical thinking skills are executed by System 2, and that these skills also serve to monitor, evaluate, and control the judgments produced by the System 1 associational process. Hence, Figure 2 shows that System 1 judgments provide input to critical thinking skills. The two processes are thought to run in parallel and interact to produce judgments. Because System 1 is truly an automatic and uncontrolled process, it cannot be consciously initiated or stopped. For this reason, only the products, and not the process, of System 1 is monitored. Because System 1 is quick, it often comes to judgment before System 2, but System 2 may override, or confirm, that judgment. Therefore, System 2 has the potential for controlling judgment, although it may not always utilize that potential.
Critical thinking can provide a thorough examination of the problem at hand. Although System 1 might derive just one solution (Klein, 1999), System 2 can provide multiple potential solutions. System 1 works to narrow possible action paths, which is often highly effective when the task must be accomplished quickly and when the problem space is limited. However, when the problem space is novel or complex or when solutions must be innovative, critical thinking skills are more powerful. They also have the meta-cognitive capability to monitor the progress of their own processing, as represented by the selfmonitoring arrows leading out and back into the System 2 processor in Figure 2.
Figure 2 also shows how the processing engines interact with environmental and individual factors. Both systems receive initial input from the environment in the form of information about a situation or problem that requires judgment. Part of that input is a meta-task that defines the general purpose of judgment. The other part of the input is information about the situation. System 1 immediately and automatically begins processing of the input by searching through its associational network for potential solutions that will satisfy the purpose. Critical thinking, motored by the System 2 processing engine, receives the same input, filtered through predisposing individual difference factors, which are discussed in greater detail below. If critical thinking skills are engaged, they will begin to evaluate solutions offered by System 1 or they will apply deliberate reasoning to the problem.
Whether or not critical thinking is utilized depends on a variety of factors, including individual predisposition and situational variables. The sum value of these factors provides the impetus to engage in effortful critical thinking, but that motivation must exceed some threshold value. In the paragraphs below, each component of the model is examined in more detail.