Despite their somewhat differing philosphical views, one thing that Socrates, Plato, and Aristotle all shared in common was a great disdain for the Sophists. The Sophists were a professional group skilled in the art of rhetoric who taught debating skills, famous for being able to argue either side of any given issue persuasively.
What irked the three aforementioned philosphers a great deal was the Sophists purported ability to "prove" any position. Plato and Aristotle viewed sophists very much as the ancient equivalent of the bullshitter, a person to whom truth is incidental to his purposes. Indeed, it is through the works of Plato and Aristotle that the negative connotation of the word sophist has come into common usage.
In the dialogue Gorgias, Plato, using Socrates as a mouthpiece, questions Gorgias (who is based on a well known contemporary Sophist of the same name) about the nature of rhetoric. Gorgias admits to Socrates that the purpose of rhetoric is persuasion
After this admission Socrates begins to ask Gorgia if there is not any other means of persuasion besides rhetoric, and if there is a difference between "having learned" and "having believed." The point Socrates is making is that there are two forms of persuasion: one which leads to knowledge , and one which leads to belief without knowledge.
Up until this point Gorgias had maintained that rhetoric was the most noble of arts. Here Socrates begins to challenge that claim. The ultimate point that Plato seeks to make through Socrates is that the rhetoric of the Sophists, by its creation of belief irrespective of truth, is an ignoble art. Plato draws this charge out of his metaphysics, in which he believes the good is the pursuit of truth, to which Plato means a higher plane of transcendent reality (Plato's cave parable illustrates this view.)
While I don't think Plato's metaphysics has much merit - it was through the early Church fathers absorption of Plato's metaphysics (via Plotinus) that the notion of a "higher" Heavenly plane of reality, more real than this one, came to be a part of Church doctrine - I don't think one needs accept Plato's metaphysical beliefs to appreciate his criticism of rhetoric directed at getting someone to believe something irrespective of its truth value.
Apparently, bullshit was as irksome in Plato's day as it is now.
Blogger's Note - While the Sophists were heavily criticized, they were not completely without merit. In a future entry I'll explain what the Sophists got right.