Here there seems to be at least a trace of the linguistic thesis of philosophical behaviorism as exemplified by Carnap and, at one time, Hempel. The idea seems to be that the mentalistic statements have the same meaning as the behaviors that count as evidence for them. Unless there is a weak spot in our causal chain so that the second link is not lawfully determined by the first, or the third by the second, then the first and third links must be lawfully related. At first sight, this looks very reasonable. Dennett's computer analogy, which I mentioned above, is helpful here.
Mentalistic explanations are homuncular.
Skinner in a number of places objects to mentalistic explanations that they in effect invoke a little person or homunculus with all the same abilities that the ordinary person has. There's clearly something to this. But notice two things. But it doesn't follow that homunculi are useless. They may nevertheless accomplish something if they are dumber than the original person.