ABSTRACT
This chapter considers the role of congruity and incongruity and other 'active principles' in Robert Hooke's efforts to develop a complete system of natural philosophy. Active principles are to be understood as 'occult qualities' of bodies which were invoked by natural philosophers to account for particular kinds of observed physical activity. Congruity and incongruity were the first of such qualities to make an appearance in Hooke's natural philosophy. Ron Millen has endorsed Keith Hutchison's thesis while arguing that the beginnings of the reassessment of occult qualities can be traced back to a number of Renaissance Aristotelian or Galenic thinkers. Robert Hooke's positive attitude towards the traditions of natural magic is clearly brought out in his Cutlerian Lecture 'Of Dr. Dee's Book of Spirits'. If Isaac Newton can be said to have drawn on magical traditions, one of his sources for those traditions was surely Robert Hooke.
