 The principle of behavior is an empirical generalization. Basically, it just means that it's a statement describing a functional relation between a behavior and one or more of its controlling variables. The generalization piece means that it's either going to go across many settings, across many species, across many individuals, so on and so forth. A principle of behavior would be habituation. We understand how organisms habituate. A principle of behavior would be reinforcement. We understand how people are reinforced. Organisms are reinforced. A principle of behavior would be extinction. A principle of behavior would be motivating operations. You get the idea here, folks. All the stuff that has been well researched and has experimental evidence across many species, usually, and across many settings, environments, and contexts, and all that other fun stuff leads us to the principle of behavior.