What principle states that to be perceived as different, two stimuli must differ by a constant minimum percentage?

The principle that states two stimuli must differ by a constant minimum percentage in order to be perceived as different is known as Weber's Law. This law, formulated by Ernst Heinrich Weber in the 19th century, quantitatively describes the just noticeable difference (JND) between two stimuli. According to Weber's Law, the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus. This means that, for example, if you have a weight of 100 grams, a 2-gram increase might be noticed, while a 20-gram increase might not be noticed at a higher original weight, demonstrating that it is the proportional increase that matters, rather than an absolute change.

This principle is foundational in understanding sensory perception and highlights the importance of relative difference in human perception. It is especially applicable in scenarios involving sensations like weight, brightness, and sound, where the perception of change is not solely dependent on the absolute value but rather on the context and ratio of the stimuli involved. Understanding Weber's Law helps to explain how sensory thresholds function in various contexts within psychology.