The concept of "System 1" and "System 2" were created by Israeli-American Nobel Laureate and psychologist Daniel Kahneman, Ph.D. He uses "System 1" to refer to the emotional, intuitive part of our minds, while "System 2" refers to the analytical and controlled part of our minds. Kahneman believes that these different systems lead to our use of heuristics, or mental shortcuts, and cognitive biases because System 1 is what he refers to as a "storyteller" (see the APA Monitor article with an interview of Kahneman at the link below). That is, System 1 likes to make up a coherent story using the information readily available to it. This system, as it is highly intuitive, does not pause to realize the information that it has left out. Instead, it relies on the idea of what Kahneman calls "WYSIATI: What You See Is All There Is."
For example, we make snap judgments about others based on limited information, such as their appearance, without stopping to consider what we don't know. This is how we often practice bias. In addition, we might rely on heuristics such as "anchoring and adjustment" (see the list of heuristics below). In this heuristic, one assumes there is a given starting point from which one can operate. An example is a salesperson starting at a very high quote and going down from that quote, leading the buyer to believe he or she is getting a good deal when he or she isn't. In order to check System 1, we need to employ System 2, or our deliberate, analytical side, to ask what we are missing. We need to determine when we are not seeing the whole story and resist only relying on System 1, which is the more automatic and easier system to use.