This text looks at the influence on understanding, insight, expertise and the advance of knowledge of the forms of representation we use. It addresses questions such as: is there any such thing as (FOR-free) data?; can FORs be complete or are they always limited?; if they are limited, do we then have pluralism, ie the whole truth needs more than one FOR?; if we have pluralism, do we have relativism?; if we are not relativists, ie we think one FOR is not as good as another, by what criteria do we evaluate a FOR? This book should be of interest to those involves in the fields of cognitive science and artificial intelligence, and psychology and philosophy.