A successful new theoretical idea typically alters and extends the existing body of theory to allow for observational facts that could not previously be understood or incorporated. It also makes possible new predictions that can some day be tested. Almost always, the novel idea includes a negative insight, the recognition that some previously accepted principle is wrong and must be discarded. (Often an earlier correct idea was accompanied, for historical reasons, by unnecessary intellectual baggage that is now essential to jettison.) In any event, it is only by breaking away from the excessively restrictive received idea that progress can be made.