  The Theaetetus is an extended attack on certain assumptions and intuitions about knowledge that the intelligent man-in-the-street—Theaetetus, for instance—might find initially attractive, and which some philosophers known to Plato—Protagoras and Heracleitus, for instance—had worked up into complex and sophisticated philosophical theories. Basic to all these assumptions and intuitions, which here have been grouped together under the name “empiricism,” is the idea that knowledge is constructed out of perception and perception alone.