Lying and deception are distinct activities. If they are permissible, then they are permissible for at least one of two reasons: whether or not we have the right to know something and whether or not we can weigh knowledge against life. Deception may be permissible, but further investigation is required.
Deception is an activity that includes lying, but does not require that we say anything we know to be false. Deception is simply an act that intends to mislead someone else about the truth. Some deceptive activities are not even spoken or written. Lying intends to mislead someone else about the truth and it also claims something that we know is false. For example, consider a situation in which you are asked to clean the kitchen. You go to the kitchen and notice that it is clean. So you tell the person that asked you to clean that the kitchen is clean. You know that he will believe you cleaned the kitchen. That is why you said what you did. You deceived him, but you did not lie.
There are two possible justifications for deception. The first justification is that there is some kind of right that gives you either a duty or permission to deceive. Such a right would have to be right to prevent others from knowing the truth about something. That right could only exist if there were some good that the right was protecting. For example, right to free speech protects speech because it leads to truth. So a right to deceive would have to protect some kind of good. The one most commonly used is the right to life. If us telling the truth will lead another to death, then we have the right to prevent the truth from being known. Therefore, this justification collapses into the second justification.
The second justification is that of life. Deception is permissible to save someone’s life. If deception is not permissible for this reason, then it is never permissible. Philosophers have decided this issue in a number of ways. Some have said that deception was permissible but lying is not. Others have claimed that lying is permissible too. Still others believe that deception of any kind is always wrong.
If deception is permissible, then the right to deceive must exist alongside the right to free speech. The right to free speech exists because of the value of knowledge. So speech that is contrary to knowledge automatically works against the right to free speech. Now if the speech we say is true, but deceptive then the speech does not itself work against the right to free speech. In the particular context, that speech is in fact contrary to knowledge, even though the speech itself is not contrary to knowledge. But knowledge is always known by people. The right to free speech and the good of knowledge cannot be contrary to life either. In fact, they depend on it. So it seems plausible to claim that it is sometimes permissible to deceive. Further investigation of these claims is necessary to be sure though.
Next, I will discuss using speech to harm.