The development of positivism, which had a considerable influence on the evolution of psychiatric thought and practice during the second half of the nineteenth century, is outlined. It was within this intellectual framework that figures such as Kraepelin, Bleuler and Schneider developed psychiatric nosologies and diagnostic criteria for certain mental illnesses. While there was little scientific evidence to support the claim that medical treatments had any beneficial effects on psychiatric disorders, nevertheless psychiatric institutions were established in the mid-nineteenth century based on the medical model. Nurses were expected to observe, collect and report data on mental patients which were then presented to doctors for analysis. The intellectual climate of the asylums was such that nurses were not encouraged to question the scientific principles upon which the therapeutic regime was based, nor were they encouraged to seek a rationale for their daily observations and data collection. The specialized training for asylum nurses which was introduced towards the end of the nineteenth century did not give nurses their own professional identity, but rather reinforced the supremacy of medical knowledge in the care of the mentally ill. Trained nurses enhanced medical credibility, but did not progress care of the mentally ill because their training did not imply or encourage questioning of the positivistic basis of psychiatric treatment.