When you take an addictive drug for the first time — nicotine, let's say — a new agent begins to bud around that source of pleasure (i.e., the neurotransmitters that flood your brain while smoking). The agent starts out small and weak. But the more you feed it, the bigger it grows, until there are many neurons, many modules, and even other brain-agents under its influence, feeding off the nicotine and craving it in ever larger doses, co-opting your planning and reasoning skills so it can scheme about how to get more of it.
This process, of course, is extremely adaptive for us, as evolved organisms — but only when the pleasure corresponds to something of survival or reproductive value: food, sex, social status, mastery of physical skills. The fact that our brains are capable of growing agents dedicated to pursuing food and sex is essential to our survival. It's only in the modern (super-stimulating) environment that we get into trouble.
This American Life did a nice segment on addiction a few years back, in which the producers — seemingly on a lark — asked people to personify their addictions. "It was like people had been waiting all their lives for somebody to ask them this question," said the producers, and they gushed forth with descriptions of the 'voice' of their inner addict:
"The voice is irresistible, always. I'm in the thrall of that voice."
"Totally out of control. It's got this life of its own, and I can't tame it anymore."
"I actually have a name for the voice. I call it Stan. Stan is the guy who tells me to have the extra glass of wine. Stan is the guy who tells me to smoke."
Note that this isn't literal speech, as in an auditory hallucination. Instead, the 'voice' is simply an agent whose influence is accessible to introspection, and thus capable of being put to words, as an imaginative/interpretive gloss. That we call them voices is simply a testament to the high level of abstraction at which these agents operate.
It's this same sense — abstract, non-explicit — in which these agents engage in 'reasoning,' 'negotiation,' 'bargaining,' joining 'alliances,' and other forms of coalitional politics. When two sub-personal agents are bargaining, for example, they're not using words to do it, but the process is nevertheless the kind of thing that can be put into words — and thus these agents can be very 'persuasive'. Again here's This American Life:
[Over-sleeper] "Then I'll get up five minutes later and [the voice will] be like, 'Eh, I mean, you don't need to iron a skirt. Do you really need to iron the skirt? If you need to iron the skirt, do you need to be wearing the skirt? Maybe you could wear a different skirt, and then you could sleep for 10 more minutes.' And that seems like a reasonable negotiation."
Obsessions, compulsions, addictions, and other "inner demons" aren't the only agents with real power to control and explain our behavior; our brains are host to 'benevolent' agents as well. Our consciences, for example. These are agents that live inside our brains, who are being trained throughout our lives, but especially in childhood, by our interactions with parents, authority figures, and other moral teachers, and by various rewards and (especially) punishments.
Certain religious communities, such as the evangelicals studied by Tanya Luhrmann, spend a great deal of time and effort teaching themselves to 'hear' the (metaphorical) voice of God, or to interpret His will. "People train the mind," she says, "in such a way that they experience part of their mind as the presence of God." This 'God' is nothing more and nothing less than an internalised, personified agent representing society's interests.
It's an interesting feature of our brains that society (or perhaps "elite society") can install these types of agents — God, the conscience, a sense of morality — to look after its own interests. This is reminiscent of the way the UN will install weapons inspectors or election observers inside otherwise-sovereign nations.
