- Psychophysics is part psycho and part physics. Sensory systems do not simply record the physical world (like an audio or video recorder). The world appears to us the way it does because: (1) we perceive only within the limits of our nervous system, but (2) our nervous system has evolved to reflect certain aspects of the world very accurately. 
- Color matching is the single best psychology experiment of all time. Simple, lawful behavior cries out for a simple explanation. Color matching and trichromacy are the basis for generations of color technology and engineering: textiles, color printing, color TV, color reproduction on the internet, color reproduction with digital cameras/photoprinters, etc. Psychophysics rules! Most of our theories about how perception works come initially from behavioral/psychophysical studies. For example, to check the accuracy of their primate cone spectral sensitivity measurements (accurate to 1 part in a million), Denis Baylor and colleagues compared their measurements to human color matching functions, measured psychophysically decades earlier. 
- Theoretical tool: linear systems.
- Empirical tool: matching.
- Computational principles: linear summation, univariate/static nonlinearity (e.g., sigmoid), adaptation, parallel pathways.
- Principle of univariance. If two lights evoke the same responses in the three cone types, then the two lights will look the same. All that matters is the excitation in the three cone types. Each cone outputs only a single number (i.e., satisfies univariance). It tells us how many photons it has absorbed, but nothing about which photons they were (i.e., which wavelength). Applies to other domains as well (to any population or subpopulation of neurons performing any perceptual or cognitive function).
- Perceptual constancy (e.g., lightness constancy, color constancy, size constancy, etc.), adaptation, and visual illusions (e.g., simultaneous color contrast, chromatic aftereffect, visual motion aftereffect, etc.).