I was dialoguing with someone and he stated that “truth is just a matter of one’s own perception.”

He explained with an illustration. Take a glass of water which is half full. One person looking at the glass may say it is half full, and another person looking at it may say it is half empty. Which person is right? That was his line of argument.

Please show me how to answer something like this.

I have addressed relativism (and subjectivism, etc.) in several places in my books, so you should review what I have already written on the subject. You should also read my Apologetics in Conversation to learn how to deal with people in conversation.

A basic response to relativism is that it is self-refuting. If he says that “truth is a matter of perception,” then even this statement is only a matter of perception, so that it cannot be universally true that truth is a matter of perception. In other words, that truth is a matter of perception is nothing more than the person’s perception. It does not mean that it is necessarily the case, and it does not mean that you have to accept it.

Then, the illustration consists of statements that are too ambiguous to prove his point, since they leave out vital information such as the reference point and the objects being considered, but once you insert the missing information, the statements become clearly absolute. That is, considering the full capacity of the glass, half of it contains water, and half it contains non-water (let us just say air). I am referring to only the water when I say, “The glass is half full,” and I am referring to only the air (non-water part) when I say, “The glass is half empty,” but both are absolute statements.

The claim is also sophistical. You mean something definite and different by “truth” (X) and “perception” (Y), and all he does is to change the meaning of “truth” so that it attaches to Y rather than to X. In other words, he is saying, “The word that you use to designate X should be used to designate Y instead.” But then, what about X? Is there such a thing as X or not? Is X coherent or not? He evades this matter of truth without refuting it. In effect, his illustration merely explains to you what he means by Y, rather than refuting your conception of X.

It is possible to change the word “car” so that it now refers to a bicycle by declaring, “a car is just a bicycle,” and then you can describe a bicycle to illustrate your meaning, but this has nothing to do with whether or not there are four-wheeled motorized transportation devices. Taking away the word “car” from you does not actually take away the car.

Moreover, once he has affirmed his premise, that “truth is a matter of perception,” from now on everything that he says should be taken as just “a matter of perception.” This is the logical consequence of his philosophy. You should not only tell him this, but you should really act like it and treat him by this standard in all your conversations and interactions with him.

That is, argue with him according to the logical implications of his philosophy, and then treat him according to its practical implications. Insist on this even when there will be serious or even dangerous consequences for him, for example, in matters that have to do with money, legality, or safety, and each time, remind him that you are only following his standard. He must either surrender, or suffer the consequences of his own philosophy.

Although I will be the first to tell you that only logical consequences matter in intellectual debate, and that practical consequences never amount to a logical refutation, he should indeed endure these consequences if he stands by his philosophy of relativism. If he will not heed rational arguments, perhaps this non-rational (practical) means of persuasion will make him reconsider the rational merits of his position. On the other hand, since he has not proved his premise (and he cannot, because any proof would just be a matter of perception), and since you have not affirmed it, the things that you say do not need to be taken as just a matter of perception.

Depending on his attitude and response (he might not surrender so easily), you might need to shock and offend him. So, with tape recorder in hand, ask him to restate his premise, that “truth is only a matter of perception.” Then, you can say, it follows that it is only a matter of perception that his mother is not a slut and a whore, and that from a certain perspective, it is indeed “true” that his mother is a slut and a whore. Force him to admit it without evasion and qualification. After that, call his parents and play the tape back to them.

Repeat the procedure for other situations and relationships in his life. Make him admit that it is only a matter of perception that he is not stealing company property at his place of employment, and that from a certain perspective, it is indeed “true” that he is stealing company property. Then, call his employer and play him the tape.

Then, make him admit that he is an adulterer and that his wife is an ugly pig (as a matter of perception, of course), and then play the tape back to his wife. Make him admit that he is a murderer and a rapist, and that he wants to murder and rape his own children (again, as a matter of perception), and then play the tape back to his children, or also to everyone who knows him.

Of course, you should tell him that you are recording him and what you are going to do with the tape, giving him a chance to recant his philosophy. You are not trying to trick him, or to make him admit anything that is contrary to his own explicit philosophy. You are not really the one saying these things (since you deny that truth is just a matter of perception), but you are asking him whether these are some of the things that he would say, as he should, given his philosophy. He ought to be fine with the consequences, brought upon him by his own philosophy. Perhaps he should defend himself to those he thus offends by teaching them relativism.

If anything bad happens to him, he has done it to himself by his philosophy. If trouble befalls him because of this, then it is still just “a matter of perception” that these consequences are undesirable. He does not have to be a relativist, and he can surrender at any time when you confront him as above. So it is his fault if he remains stubborn.

In any case, this procedure is a pragmatic method, making his life unlivable by his philosophy, and nothing in it amounts to a logical refutation of relativism. Thus, even if he surrenders under these circumstances, it does not mean that you have logically refuted relativism by pragmatism, since pragmatism cannot refute anything. However, by employing this non-rational method, you might successfully force this irrational person to engage you again in debate, and to reconsider the rational merits of the biblical view.