As a philosopher, I value reason. Indeed, like most people nowadays, I consider the use of reason to shape the beliefs of others to be, on the whole, a good thing, and the use of techniques like threats, brainwashing, peer-pressure and indoctrination to be a bad thing. But why should reason be preferred to these other methods of shaping belief?
Causal determination determines what will happen. For example, given the causal power of these rails to direct this train, the train will go to Oxford. Normative determination, on the other hand, determines not what will happen, but what ought to. It is a distinct category of determination.
A rational argument shows you what you ought to believe if you want to give your beliefs the best chance of being true. Take this valid deductive argument:
All men smell
John is a man
Therefore, John smells.
To recognise that this argument is valid is just to recognize that if you believe that all men smell, and that John is a man, then you ought to believe that John smells. But of course this argument doesn’t causally compel you to accept that conclusion even if you do accept the premises. You’re free to be irrational.
This isn't to deny that rational arguments have causal power. Of course they do. A good argument can have the power to change history (consider the arguments of the campaigner against slavery William Wilberforce). But when rational arguments have the causal power to shape people’s thinking, they typically have it as a result of their having normative power. People change their opinions because they recognize the normative force of the argument.
Notice, by the way, that we can easily show that a rational argument doesn’t have normative power simply by virtue of its having causal power to shape people’s beliefs. The obvious counter-example is a fallacious argument. A fallacious argument lacks normative power. But notice that, if the fallacy is seductive, it will still have considerable causal power to influence belief.
So rational arguments can and do have causal power. But that is not to say that rational argument is just another form of causal manipulation alongside e.g. threats, brainwashing, peer pressure, etc.
To sum up, we have seen that, when it comes to shaping belief, rational argument differs from these other methods in at least two important ways:
(i) reason is truth-sensitive (whereas purely causal mechanisms typically are not)
(ii) while rational arguments can be causally powerful, their causal power typically derives from their normative power – which is a distinct, non-causal form of "power".