A mental block is an uncontrollable suppression or repression of painful or unwanted thoughts/memories. It can also be an inability to continue or complete a train of thought, as in the case of writer's block. In the case of writer's block, many find it helpful to take a break and revisit their topic. Another tactic that is used when people with mental blocks are learning new information is repetition. A similar phenomenon occurs when one cannot solve a problem in mathematics which one would normally consider as simple.
