Science, not rule-of-thumb: According to this principle, there should be scientific study and analysis of each element of job, in order to replace the old rule of thumb approach or trial and error method. Rule of Thumb means which is not based on exact science or exact measurement. This principle encourages thinking before doing. Taylor emphasised that manager should scientifically analyse each component of work and the motions required to perform it. Scientific methods are based on cause and effect whereas rule of thumb means absolute rule of manager in taking managerial decisions.