History quiz

Who was the skeptic who believed communication is impossible and responded to opponents by simply wagging his finger?

The skeptic who believed that communication is impossible was Gorgias. He responded to opponents by making a "symbolic gesture": wagging his finger. He believed that there are three main reasons why communication is impossible: 1) Nothing exists; 2) If anything existed, it would be unknowable; and 3) Even if it were knowable, it could not be communicated.