The duration of pain is key, and in the research literature, a distinction is made between chronic and acute pain. Whereas acute pain is characterised by recent onset, limited duration, and a clearly identifiable cause chronic pain is long-term (lasting more than 3 months), is not clearly related to a specific injury, and may not even have a clear cause. Henry Ward Beecher believed that this mismatch between the severity of injury and pain was due to the soldiers' relief that their injury allowed them to leave the battlefield behind. While escaping the dangers of the battlefield must have been a positive experience for these soldiers, this analysis does not explain the underreporting of pain and injury. In response to attempts to understand why the intensity of a pain source does not always correspond with a person's reported pain, theories needed to become more sophisticated.