Which principle states that the smallest detectable difference between two stimuli is a constant?

Weber's Law articulates that the smallest detectable difference between two stimuli, known as the just noticeable difference (JND), is a constant proportion relative to the original stimulus. This principle suggests that for perception to notice a difference, the change in stimulus must be a certain fraction or percentage of the original stimulus intensity. For example, if you're lifting weights, the increase in weight that you can detect varies depending on the original weight; a heavier weight requires a larger absolute increase to perceive a difference compared to a lighter weight. This reliability in the relationship between stimulus intensity and perceived difference helps psychologists understand sensory perception quantitatively.

In contrast, Fechner’s Law expands upon Weber's Law by suggesting that perceived intensity increases logarithmically as stimulus intensity increases, rather than solely through ratios. Stevens' Power Law relates perceived magnitude to the actual intensity in a more complex way, while Signal Detection Theory focuses on distinguishing between signal and noise in the presence of uncertainty, not on the proportional relationship of changes in stimulus intensity.