The dual scale model of weighing reasons

The metaphor of weighing reasons brings to mind a single (double-pan balance) scale. The reasons for φ go in one pan and the reasons for ~φ go in the other. The relative weights, as indicated by the relative heights of the two pans of the scale, determine the deontic status of φ. This model is simple and intuitive, but it cannot capture what it is to weigh reasons correctly. A reason pushes the φ pan down toward permissibility (has justifying weight) only insofar as it pushes the ~φ pan up toward impermissibility (has requiring weight). Thus, Single Scale is committed to Single Proportion (first pass): all reasons have the same proportion of justifying and requiring weight. Many normative theories reject Single Proportion, and they are not thereby confused about how to weigh reasons. One can be mistaken about the weight of a reason without being confused about how reasons are to be weighed. Single Scale is problematic, not because it appeals to the image of a scale, but because it appeals to the image of a single scale. We need two scales to capture what it is to weigh reasons correctly.