The senses and nervous system notice differences.

"In the branch of experimental psychology focused on sense, sensation, and perception, which is called psychophysics, a just-noticeable difference or JND is the amount something must be changed in order for a difference to be noticeable, detectable at least half the time. This limen is also known as the difference limen, differential threshold, or least perceptible difference."

The Just-Noticeable Difference JND

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where Pasted Graphic 1 is the original intensity of the particular stimulation, I is the addition to it required for the change to be perceived (the JND), and k is a constant. This rule was first discovered by Ernst Heinrich Weber (1795–1878), an anatomist and physiologist, in experiments on the thresholds of perception of lifted weights.

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