People don’t always say what’s on their minds. One reason is that they are unwilling. For example, someone might report smoking a pack of cigarettes per day because they are embarrassed to admit that they smoke two. Another reason is that they are unable. A smoker might truly believe that she smokes a pack a day, or might not keep track at all. The difference between being unwilling and unable is the difference between purposely hiding something from someone and unknowingly hiding something from yourself.