For a fact to be incorporated into science, it must be verfiable — other people have to be able to observe it. This criterion presented an especially difficult problem for psychology in its early years because psychology was initially founded to study consciousness. But it is hard to study consciousness scientifically, because it is a "private event" that cannot be verified by others. Since empirical verification is a requirement of science, how could it be included in a scientific discipline? This was one of the reasons structuralism died out and behaviorism gained in popularity.