Behaviorism” describes a range of positions, but they all claim that we should focus on “observable behavior” instead of mere mental states.  (For more, see here, here, here, and here).  While this position is counterintuitive, it seems to work: One of the easiest ways to change the behavior of an animal – or a human – is to reward the behavior you want, and punish the behavior you don’t want.  You don’t need a lengthy exchange of ideas; just deploy your sticks and carrots, and watch them do their magic.  If you want your child to sleep through the night, for example, let him cry for a while.  Once you stop giving attention to kids who don’t sleep, the problem usually solves itself.

If you’re paying attention, though, you’ll notice a critical act of hand-waving.  How do you know that “attention” is a reward, not a punishment?  Indeed, how do you know that a carrot is a reward, and a stick is a punishment?  If you’re trying to motivate a carnivorous masochist, you should use the stick when he pleases you, and the carrot when he doesn’t.  Less obviously, but equally critically, the behaviorist usually tries to impose reinforcement soon after the desired/offending behavior.  But not always.  If he’s dealing with a rat or a toddler, he’ll want immediate consequences.  If he’s dealing with a banker, he might confidently punish bad behavior with a monetary fine a year after the incident.

The lesson is that even the staunchest behaviorist makes implicit assumptions about subjects’ mental states.  He makes assumptions about their preferences and emotions, such as: animals like food, kids like attention, adults like money, and most creatures aren’t carnivorous masochists.  He makes assumptions about their beliefs, such as: rats and toddlers won’t think X is a punishment for Y unless X happens soon after Y, but bankers can see more subtle connections.  Behaviorism works because (a) animals and people do have preferences, emotions, beliefs, etc., and (b) common sense gives us lots of insight into what these preferences, emotions, beliefs, etc. typically are.  Behaviorism works, in short, because it’s false: We already know enough about thoughts and feelings to design rewards and punishments that animals and humans grasp and care about.

P.S. I wish this point were original to me, but it’s not.  I first heard it in high school while reading The Ayn Rand LexiconSurprisingly successful Randian psychologist Edwin Locke explains it elegantly:

Behaviorism’s substitute for the mind is certain entities in the environment
called “reinforcers.” A “reinforcer,” say the Behaviorists, is an event which
follows a response and makes subsequent responses of the same type more likely.
“What type of events change the probability of responding?” we ask.
“Reinforcing events,” we are told. “What is a reinforcing event?” we inquire.
“One which modifies response probability,” they reply. “Why does a reinforcer
reinforce?” we ask. “That’s not a relevant question,” they answer. . . . To
understand why a “reinforcer” reinforces, Behaviorists would have to make
reference to the individual’s mental contents and processes–i.e., they would
have to abandon Behaviorism.