In modern society, we often tend to study the individual in the context of the individual, looking at the differing desires, needs, and cognitions that influence each and every one of us. In the Middle Ages, this was not necessarily the case, and the hardships faced by Europeans led them to look at individuals as part of a wider picture, based in the battle of good against evil. Mental illness was not seen as something that affected an indivdual, but as a condition that played a role in humanity's conflict between virtue and vice.
After Augustine, wider events affected the study of the human mind and of science in general. Science and philosophy usually prosper in wealthier societies, because an affluent society can afford to let philosophers, scholars and poets enrich culture. Conversely, a poorer society is concerned with more pragmatic things, and the tribulations of the Early Middle Ages acted as a breeding ground for superstition rather than science.
After the fall of the Roman Empire, at the end of the 5th century, the works of scholars laid forgotten as humanity stopped looking for models and trying to understand the natural processes of the physical world. Instead, they turned to magic and superstition, searching for supernatural explanations for phenomena they did not understand.
The Visigoths, Vandals, Huns and other barbarians laid waste to Europe, and a plague in the Sixth Century finally destroyed the Roman dominance of Europe. In a changing, increasingly brutal world, as is often the case, religion came to dominate as people flocked to the idea of salvation after life, some hope to provide purpose to the suffering present on earth. Allegory and biblical explanations began to replace reasoning and curiosity as people sought entry to the afterlife. This affected psychology and the study of the mind, as many mental disorders, notably depression and anxiety, became tied with sin and demonic possession.
The Early Middle Ages was a morbid society, with death and eternal torment reflected in the attitudes and the art of the time, including paintings and the sculptures adorning cathedrals. The fear of the end of the world was a prevalent theme, and we can postulate that this led to some of the societal mental conditions present at the time, such as self-punishment, asceticism, self-flagellations, pilgrimages, and processional worship. Others demonized non-Christians, especially Jews and Muslims, blaming them for the series of torments sent by God.
Reality was seen as a hierarchy flowing from creator, down through angels and humanity, to the lowest intelligence and inanimate objects of creation. Mental illness was seen as a disorder in this progression. Nemesius, Bishop of Emesa, in Syria (c. 390CE), wrote On the Nature of Man, promoting this view, which dominated Medieval thought. People were rarely seen as individuals, rather as part of a collective fighting for salvation or damnation.
The domination of allegory and symbolism in European thought filtered into art, and scholars, dominated by clerics, began to talk of man as a whole, rather than individuals, seeing humanity as the battleground between virtue and vice, good and evil. Thus, mental illness was not defined by its effect on an individual person but as a condition, affliction, or punishment.
Because people were rarely seen as individual personalities, there was little discussion of mental illnesses, although it must be emphasized that the theoretical writings of Augustine and other scholars told us little about how people were actually treated, as most sufferers would belong to the lower classes.
It would be extremely unfair to claim that Early Middle Age society was completely insensitive to medical needs, even mental. Many monasteries provided medical facilities for their own members, pilgrims, and travelers, and the Byzantine Eastern Church actually used these to treat the poor and the crippled, as part of their charitable duties. Indeed, in the Sixth Century, the hospital set up by the monk, Theodosius, near Jerusalem, may have had a section for the mentally ill. This appeared to be unparalleled in Western Europe.
Otherwise, little is known about psychology and psychiatry in the Early Middle Ages, with virtually no information surviving, especially concerning the poorer classes and rural populations. It is likely that the mentally ill were kept at home, supported by friends and community and treated with herbs and other folk remedies. Some may have been considered to be possessed by demons, and exorcised.
Magic and contagious disease were seen as the root of mental disturbances amongst the populace, so a combination of amulets and other magical items drove out evil magic or spirits. The Church continuously condemned such pagan practices, but they persisted. As an example, epilepsy was often treated with saintly relics, and people travelled huge distances to various shrines and holy places.
Priests played some role in treating the sick, and this probably included mental conditions. As stated in many books, originally written in the monasteries on the fringes of the Celtic World, notably Ireland and Wales, priests tried to cure like with like, so they might combat despair with hope, and aggression with peace. As part of a confessional, priests may have become therapists with some insight into the human mind gained through experience. Throughout history, the idea of the shaman or priest as spiritual healer, including the mind, is a common one. This is likely to have been the case in the Early Middle Ages.