Here's a really short (and necessarily over-simplified) version of her argument:
- Why is it wrong to lie? It seems obvious that it must be but why?
- Many lies cause little or no harm and some lies can even do good.
- But what if everyone did it?
- Well, if everyone did it then trust would break down and nothing would work.
- Therefore it requires that large numbers of people tell the truth in order for human interaction to work.
- The minority who insist on lying are, therefore, free riders.
- Free riders here being an economic concept to designate those who rely on everybody else to be honest while they cheat. (Think of the person who never refills the ice tray confident that since the rest of the family all do their share it won't matter.)
- To be a free rider is to exploit the rest of society and therefore wrong.
We then get the harm principle (#2). This narrows down our options because the only moral reason available to us is real or potential harm to others. All other options such as that it might be better to tell the truth because it builds character are closed from the beginning.
What if everyone did it (#3) is John Stuart Mill's argument. It takes moral concern away from the individual and makes it a societal question. This further shuts the door on the notion that there might be any reasons of personal moral growth involved.
Worse though is that Bok's answer (#4) is just wrong. Everyone does lie and yet language and meaning is no problem at all. Carpentry is only possible because wood is relatively stable and language is only possible because most people tell the truth most of the time but lying is built right into the foundation of our language. There would be no use for the word "lie" if we always told the truth nor would there be any use for related expressions such as "I swear to tell the truth." The very existence of this sort of option in our language tells us that lying is always an option.
It does take the existence of large numbers of people (#5) who tell the truth in order for the system to work but this is too simple. Truth telling and lying are learned skills and they have to be learned in different contexts.
Let me give you an example of this. Imagine you have never been in business and now you are going into business. You've never signed a contract or made a verbal agreement. All you are bringing to this new activity is the distinction between lying and telling the truth you learned in grade school. Could you do it? Not without learning a whole lot more. There are standards for what counts as truth telling and lying in business.
There are different standards again for what counts as truth telling in court of law.
Bok's argument is not so much wrong as entirely inadequate.