John C. Wright responding to a materialist.
…Let us assume you had the power, the Thought Control Helmet, to reorganize at will any brain you came across, so that the ideas in that brain would conform to whatever conclusions and ideas you preferred. The moment you use it, you are treating people like rocks: they would for all practical purposes be inert material, robots or animals, things without any moral or human meaning to you. If you used the Mind Helmet on Trilby to make her fall in love with you, it would have no more meaning to you than if you wrote a love letter to your self and forged her name on it. It would not represent any judgment or thought or honest emotion on her part. It would be fan-fiction, but one where you put yourself in as a character and get Uhura to kiss you.
You would never discuss or debate or disagree with anyone again. Instead of the frustration of trying to make your ideas clear to them in words, you could merely zap them with the helmet-ray, and their thoughts would be whatever you wished. You could perhaps as a game pretend these robot people were real, and let them say and think whatever nature had randomly written into their brain-mechanisms, but it would be you pretending they were human. It would not even be a game. It would be a pastime, like solitaire. Their words and ideas would have no truth value to you.
But no matter how you treated other people, you could not treat yourself the same way; you would not use your Mind Control Helmet to force yourself to think certain ideas, because the ideas would have no truth-value to you if you imposed them on yourself in that fashion. I am not saying the owner of such a machine might not want to lie to himself in his own thoughts, or bury an unhappy memory–but the utility of ideas qua ideas, the usefulness we seek from the process of reasoning, would be lost.
Ideas that are imposed on you by the helmet, if you knew they were imposed, would not persuade you that they were true. If you did not know they were imposed, but thought you had reasoned your way to their conclusion, you were merely be deceived and insane. You could no longer trust your own thoughts to be corresponding to reality. If you eliminated from yourself the desire to have trustworthy thoughts or to have them correspond to reality, or if you eliminate your awareness of what you had done to yourself, at that point you are a muppet.
The reason why materialism is self-contradictory is that you are in effect telling me that your thoughts are controlled by a Mind Control Helmet that runs without an operator, merely Mother Nature blinding sending out unintentional thought-control-signals. But, if you actually believed that, you would conclude that your thoughts have no truth value.
Whether John C. Wright is speaking on the Space Princess movement or materialism he is always fun to read and his recent series of posts responding to materialism are quite good.