William of Occam or Ockham (1285-1348)was a 14th Century British philosopher, logician and Franciscan friar who taught at Oxford University. He was the founder of nominalism, a philosophical posture that insisted on the need for empirical demonstration of truth, and held that usually wrong deductions were made when those were based on beliefs, a matter of faith, belonged to mystical and religious realms (not to rational and critical thinking), and that were never universally verified. The reason this principle is so useful in psychology is that there is such a tendency to hypothesize in terms of belief rather than in terms of evidence. This leads to erroneous assumptions about what works and why. A full article on the types of errors that can occur when we stray from these principles can be found on www.bmsa-int.com. if you're interested in reading more about a more scientific approach to the development of psycholgical interventions. Best wishes Christine