A theory explains the mechanisms through which causes create effects.
I’m not prone to violence, but if you really want to see me punch someone, find me on a really bad day, and then tell me “Anything is ‘theoretically possible.’” While the infinite possibilities provided by theory is a common idea is society—and one you will hear expressed in freshman philosophy classes—it represents a fundamental misunderstanding of the role of theory. Another version of this same idea is “Well, that’s just a theory” as rebuttal to scientific claims.
Theories explain causality. If you believe hard work in school causes good grades, because when you work hard in school you learn more, and more learning reflects in better grades, you have a theory. Without a theory all you have are observations about reality. You may even have observations that consistently correlate, but you need a theory to explain why.
Laws are consistent observations without causality, and can be explained by different theories. You do not need a theory to realize that things fall. Aristotle’s theory of gravity was that everything had an inherent nature, so some things moved toward the center of the universe (i.e. down) whereas other things moved away from the center of the universe. Newton also had a theory: gravity is caused by inherent attraction between objects with mass. We still accept the law of gravity, and Newton’s theory, while rejecting Aristotle’s theory.
Theories must be inherently bounded to be useful. If anything were truly possible theoretically, it would be a useless theory. Hard work yields good grades implies lack of hard work does not yield good grades, or at least is indeterminate. There are inherent assumptions in the “grades” theory, including that work creates learning, grades reflect learning, and that one is in a graded environment. People who work with theories understand these constraints, but casual observers become confused when an expert says “That is theoretically possible, but in practice I am unsure,” and interpret it to mean that theoretical possibilities are broader, perhaps infinitely so, than empirical possibilities. Theoretically possible outcomes may be practically impossible because empirical realities do not match theoretical assumptions. Theoretical possibilities are actually more restrictive than practical possibilities, being contingent on the theories implied and expressed assumptions.
Good theories balance “elegance” with “parsimony.” The most elegant theory would explain everything, but would account for every possible variable, becoming infinitely complex. Parsimonious theories sacrifice elegance for simplicity, and consequently cannot explain some empirical outcomes. Simple gravitation cannot explain why airplanes fly; we have to rely on different theories to explain that. Nevertheless, gravity is a useful theory because of what it does explain.
Theories are never fully settled, but are not subject to revision because we do not like what they predict. Theories should be accepted, rejected, or revised based on empirical observation, and not how much we like them or not. Since new information constantly emerges, no theory is every completely safe. Indeed, Aristotle’s explanation stood for millenia before being revised in the early modern period. Newton’s physics were revised in the early 20th century. Sometimes we do not like what a theory implies, and while unfortunate, is insufficient reason to abandon the theory. Human attachment or aversion to certain theories, however, can also serve as an otherwise important motivator for scientific innovation, so try not to be too hard on people who seemingly reject what you believe obvious.