This paper provides empirical evidence that challenges the view that methods of clinical assessment and decision making should not rely solely on logical positivist approaches. Whilst the National Health Service (NHS) Executive currently takes a hard positivist line on what constitutes evidence-based practice, data reveal that it is not always appropriate to disregard the tacit knowledge and intuition of experienced practitioners when making assessment decisions in mental health nursing practice. Data support the case for a holistic approach which may draw on intuition and tacit knowledge, as well as traditional approaches, to meet the requirements of clients with complex mental health problems. A model based on Schon's notion of reflection in and reflection on practice is proposed which demonstrates the value of intuition and tacit knowledge. This model allows the generation of insights which may ultimately be demonstrated to be acceptable and empirically testable. It is accepted that an element of risk taking is inevitable, but the inclusion of a formal analytical process into the model reduces the likelihood of inappropriate care interventions. The cognitive processes which experienced nurses use to make clinical decisions and their implications for practice will be explored.