Fitts' law is a well known empirical relation which predicts aimed-movement time (MT) from target distance (D) and target width (W). Fitts' demonstration that MT, within limits, depends essentially on the ratio D/W implies a scale invariance that reduces the paradigm from three dimensions (MT, D, and W) to two (MT and D/W). This reduction, however, is legitimate only for narrow ranges of scale variations, a limitation that appears to have been overlooked so far. This paper advocates an explicit three-dimensional construal of Fitts' paradigm involving not only the speed (MT) and the relative amplitude (D/W), but also the absolute amplitude (D), or scale of movements. Not only is this three-dimensional description of Fitts' paradigm a technical necessity for the classic study of Fitts' law, but it paves the way for a more complete modelling of aimed-movement performance and suggests a promising adaptation of Fitts' paradigm to the recently emerged problem of target selection in zooming interfaces.