Why did Putnam argue that the statement “We are brains in a vat” defeats itself?

Hilary Putnam argued that the statement “We are brains in a vat” is self-defeating based on his theory of semantic externalism.

According to this view, the meaning of our words is determined by their causal connection to the external world. If we were actually brains in a vat, our words would not refer to real-world objects like trees or vats, but rather to the computer-generated images we experience. Consequently, if an envatted brain were to utter the sentence “We are brains in a vat,” the words “brains” and “vat” would not refer to actual brains or vats in the physical world, but to the simulated images within their experience.

Therefore, if the hypothesis were true, the statement would be false because it would not be describing the actual reality of the speaker. By demonstrating that the skeptic’s claim cannot be expressed in a meaningful way without contradicting itself, Putnam concluded that the brains-in-a-vat hypothesis is paradoxical and that skepticism is fundamentally flawed.12