Thinking Fast and Slow by Daniel Kahneman

There are two systems in our brains. One of those systems operates very quickly, the other more slowly. The first system functions automatically, intuitively, and involuntarily while the second employs more deliberation, reasoning, and concentration. The two systems can work symbiotically but quite often conflict with each other.

System 1 may employ heuristics that are inaccurate or too hastily formed while System 2 requires some effort to function correctly and is prone to error and miscalculation. System 1 is the system responsible for our intuitions, feelings, and impressions, which System 2 then turns into beliefs and calculated actions. System 2 is slow to react and System 1 is quick to react. The operations of System 2 require focus and are disrupted when attention is diverted away from those operations. System 2 can alter the way System 1 functions, to some degree. In fact, one of the key purposes of System 2 is to overcome System 1's impulses. This is where self-control comes in.

System 2, unlike System 1, can follow rules and make intelligent and deliberate choices. System 2 can also create rules based on impressions gleaned from System 1. Crucially, System 2 can program our memories so that we behave in a way that overrides habitual, instinctive, System 1 responses. System 2 is the system responsible for self-control and is the main system in charge when things get difficult. Ultimately, System 2 has the last word over System 1.

Multi-tasking that requires System 2 to simultaneously dedicate itself to more than one task is often overwhelming and unproductive. However, multi-tasking that makes use of System 1 and System 2 concurrently is more manageable and effective.