Psychophysics is the study of the quantitative functioning of perception and sensory-guided judgements. Over the last 200 years, a number of precise mathematical regularities (psychophysical “laws”) have been discovered. The investigation of psychophysical laws represents an attractive approach for neuroscience, as they constitute powerful constraints that can be used to study the neural basis of perception and decision making. We have recently made an important contribution by discovering a new psychophysical regularity — at the level of reaction time in sensory discrimination experiments in rats and humans — which subsumes Weber’s Law, the oldest and most general psychophysical regularity. We termed this regularity the Time Intensity Equivalence in Discrimination (TIED). The TIED implies that changing the overall magnitude of two sensory stimuli being discriminated (by a common multiplicative factor) is completely equivalent to a change in the units of time of the sensory discrimination process. Because the TIED is a very strict regularity, it allowed us to rule out many previously proposed explanations of Weber’s law, and to identify its necessary algorithmic ingredients. In addition, the simplest implementation of the algorithm provides a virtually complete description of the accuracy and reaction time of the subjects. We are currently using this knowledge to design perturbation and recording experiments aimed at discovering the neural basis of Weber’s law and, by extension, of simple discriminations of sensory magnitude.