Due to its subjective nature, pain can be hard to define. One system for definition of pain uses time from the onset of pain. This system divides pain into: early (acute) and late/ongoing (chronic). A middle category, subacute, is also sometimes used to assess pain. Each category is described separately. Whereas most, if not all, physicians can start to treat acute and subacute pain, chronic pain following injury is best treated by a pain specialist.