 Empiricism. This view is represented by the logical positivists. They believed that the interpretation of any scientific theory should be grounded in empirical observations. No theory, according the positivists, is cognitively meaningful unless its terms can be connected to terms that are able to express results that would verify that theory. Observational terms refer directly to observable things or observable properties of physical objects, whereas theoretical terms are explicitly defined by correspondence rules connecting them with the observational terms. Hence classical terms, like position and momentum, are exactly such terms that enable physicist to ascribe a physical meaning to quantum mechanics.