This article explores in depth one particular way of defining and measuring subject difficulty - the 'IRT approach'. First the IRT approach is briefly described. Then the analogy of using the IRT approach when the 'items' are examination subjects is explored. Next the task of defining difficulty from first principles is considered, starting from the simplest case of comparing two dichotomous items within a test. Finally, an alternative to the IRT approach, based on producing visual representations of differences in difficulty among just a few (three or four) examinations, is offered as an idea for future exploration.