Poena cullei (from Latin 'penalty of the sack') under Roman law was a type of death penalty imposed on a subject who had been found guilty of parricide. The punishment consisted of being sewn up in a leather sack, sometimes with an assortment of live animals, and then being thrown into water. The punishment may have varied widely in its frequency and precise form during the Roman period. For example, the earliest fully documented case is from ca. 100 BCE, although scholars think the punishment may have developed about a century earlier (earlier than that, murderers, including parricides, would be handed over to the aggrieved family for punishment, rather than punishment being enacted by Roman state officials). Inclusion of live animals in the sack is only documented from Early Imperial times, and at the beginning, only snakes are mentioned. At the time of Emperor Hadrian (2nd century CE), the most well known form of the punishment was documented, where a cock, a dog, a monkey and a viper were inserted in the sack. However, at the time of Hadrian "poena cullei" was made into an optional form of punishment for parricides (the alternate being thrown to the beasts in the arena). During the 3rd century CE up to the accession of Emperor Constantine, "poena cullei" fell out of use; Constantine revived it, now with only serpents to be added in the sack. Well over 200 years later, Emperor Justinian reinstituted the punishment with the four animals, and "poena cullei" remained the statutory penalty for parricides within Byzantine law for the next 400 years, when it was replaced with the punishment for parricides to be burnt alive instead.

"Poena cullei" gained a revival of sorts in late medieval and early modern Germany, with late cases of being drowned in a sack along with live animals being documented from Saxony in the first half of the 18th century.

The 19th-century historian Theodor Mommsen compiled and described in detail the various elements that at one time or another have been asserted as elements within the ritualistic execution of a parricide during the Roman Era. The following paragraph is based on that description, it is "not" to be regarded as a static ritual that always was observed, but as a descriptive enumeration of elements gleaned from several sources written over a period of several centuries. Mommsen, for example, notes that the monkey hardly can have been an ancient element in the execution ritual.

The person was first whipped, or beaten, with "virgis sanguinis" ("blood-colored rods", probably), and his head was clad/covered in a bag made of a wolf's hide. On his feet were placed clogs, or wooden shoes, and he was then put into the "poena cullei", a sack made of ox-leather. Placed along with him into the sack was also an assortment of live animals, arguably the most famous combination being that of a serpent, a cock, a monkey and a dog. The sack was put on a cart, and the cart driven by black oxen to a running stream or to the sea. Then, the sack with its inhabitants was thrown into the water.

Other variations occur, and some of the Latin phrases have been interpreted differently. For example, in his early work De Inventione, Cicero says the criminal's mouth was covered by a leathern bag, rather than a wolf's hide. He also says the person was held in prison until the large sack was made ready, whereas at least one modern author believes the sack, "culleus", involved, would have been one of the large, very common sacks Romans transported wine in, so that such a sack would have been readily available. According to the same author, such a wine sack had a volume of .

Another point of contention concerns precisely how, and by what means, the individual was beaten. In his 1920 essay ""The Lex Pompeia and the Poena Cullei"", Max Radin observes that, as expiation, convicts were typically flogged until they bled (so some commentators translates the phrase to "beaten with rods till he bleeds"), but that it might very well be that the rods themselves were painted red. Radin also points to a third option, namely that the "rods" actually were some type of shrub, since it documented from other sources that whipping with some kinds of shrub was thought to be purifying in nature.

In 1994, responding to the need for a more useful system for describing chronic pain, the International Association for the Study of Pain (IASP) classified pain according to specific characteristics:

1. region of the body involved (e.g. abdomen, lower limbs),

2. system whose dysfunction may be causing the pain (e.g., nervous, gastrointestinal),

3. duration and pattern of occurrence,

4. intensity and time since onset, and

5. cause

However, this system has been criticized by Clifford J. Woolf and others as inadequate for guiding research and treatment.

Woolf suggests three classes of pain:

1. nociceptive pain,

2. inflammatory pain which is associated with tissue damage and the infiltration of immune cells, and

3. pathological pain which is a disease state caused by damage to the nervous system or by its abnormal function (e.g. fibromyalgia, peripheral neuropathy, tension type headache, etc.).

Psychogenic pain, also called "psychalgia" or "somatoform pain", is pain caused, increased, or prolonged by mental, emotional, or behavioral factors. Headache, back pain, and stomach pain are sometimes diagnosed as psychogenic. Sufferers are often stigmatized, because both medical professionals and the general public tend to think that pain from a psychological source is not "real". However, specialists consider that it is no less actual or hurtful than pain from any other source.

People with long-term pain frequently display psychological disturbance, with elevated scores on the Minnesota Multiphasic Personality Inventory scales of hysteria, depression and hypochondriasis (the "neurotic triad"). Some investigators have argued that it is this neuroticism that causes acute pain to turn chronic, but clinical evidence points the other direction, to chronic pain causing neuroticism. When long-term pain is relieved by therapeutic intervention, scores on the neurotic triad and anxiety fall, often to normal levels. Self-esteem, often low in chronic pain patients, also shows improvement once pain has resolved.