The medieval scholars were prone, because of the faith to which they had given their whole-hearted adhesion, to see spiritual powers at work in many things. In this they were sometimes sadly mistaken, but not so much mistaken as certain generations of the nineteenth century, who absolutely refused to accept any possibility of spiritual interference in things mundane. Both the extremes are mistakes. It is manifestly more of a mistake, however, to deny spiritual influence entirely (I talk now from the standpoint of the scientist and not the believer), than to accept so much of spiritual interference as the medieval generations permitted themselves to be convinced of…The medieval peoples did much harm by accepting the position, that many persons suffering from ordinary nervous and mental diseases as we now know them were really possessed by the devil. The treatment accorded these supposedly possessed (for the moment we lay aside the question as to the possibility of the reality of diabolic possession) was not any worse than has frequently been accorded to sufferers from mental and nervous disease in presumably much more intelligent times, either because of fear of them, or neglect on account of the absence of a sufficient number of keepers, or because of curious theories of medical science.
Mankind, it is hoped, is progressing, but the amount of progress from generation to generation is not enough, that any succeeding age should criticise severely the well-intentioned though mistaken efforts of their predecessors to meet, according to the best of their ability, problems that are as deep as those involved in nervous and mental diseases.
James J. Walsh, The Popes and Science (1908)