“That’s what you think!” We’ve either heard or said this phrase at some point in our lives. But if someone were to say, “It is raining out,” and if looking outside we saw rain, would we likely retort, “That’s what you think!”?

Although what one says is a measure of his thought, we don’t employ this phrase in plain meaning or to congratulate someone on articulating his thought well; instead, we use it disparagingly. We employ it when we wish to imply that what someone thinks isn’t what is, that what he thinks exists only in a subjective manner, only within him; that he lacks objectivity, that what he thinks is but errant thought, that his thinking isn't reflective of reality. We are saying that someone is deluded; we are saying that he is wrong.

Perhaps you are thinking, “Wait, aren’t all my thoughts by nature subjective since they occur only within me?” So true! for your thoughts are not my thoughts; my thoughts, not yours. That we all are possessed of such subjectivity, objectively is true, barring this big exception: having read what I have written and understanding it, haven’t my thoughts become yours; yours, mine?

Spoken words are the symbols of mental experience and written words are the symbols of spoken words. Just as all men have not the same writing, so all men have not the same speech sounds, but the mental experiences, which these symbolize, are the same for all, as also are those things of which our experiences are the images.

- Aristotle, De Interpretatione