The document discusses three classical psychophysical methods for measuring thresholds: 1. Method of constant stimuli which measures thresholds using fixed sets of stimuli presented in random order to determine detection rates. 2. Method of limits which alternates between increasing and decreasing stimuli to find transition points. 3. Method of adjustment where the observer adjusts the stimulus level until it is perceived. These methods are used to determine absolute and difference thresholds as well as factors that can influence thresholds like habituation and expectation.