A decision journal helps you collect accurate and honest feedback on what you were thinking as you made various decisions. This feedback also helps you see when you were lucky. Sometimes things work out for very different reasons than we thought they would. The key to understanding the limits to our knowledge (see circle of competence) is to check the results of our decisions against what we thought was going to happen and why we thought it was going to happen. That feedback loop is incredibly powerful because our minds won’t provide it by themselves.
Source: How a Decision Journal Changed the Way I make Decisions (Template Included)