Articles categorized under Concepts & Methodology may discuss the basic optical principles that are necessary for understanding how light interacts both with non-biological systems (i.e., "physical optics") and with living visual systems ("physiological optics"). Other articles included here will emphasize the need for using precise language, when dealing with the topics of this website. (Casual language, or even the terminology used in many texts, may lead to apparent contradictions or circularity, whereas more careful usage would allow one to appreciate a situation with precision and clarity.) This category will also include enough background material to enable one to understand the basics of important research techniques, including the field called psychophysics – those methods by which we can study the relationships between variations in the physical world and variations in appearance.