An early theory is the Specificity theory. The Specificity Theory of Pain states that the pain experienced is proportionate to the amount of injured tissue. This theory of pain may be correct and accurate when applied to acute pain.
In dealing with chronic pain however, the specificity theory does not amply address this type of pain which requires a more delicate understanding. Another pain theory, known as the Gate Control Theory. This theory provides a more accurate perspective on chronic pain.
Phantom Limb Pain
Pain Specific Symptoms
Joint & Muscle Pain