With the widely varying theories of the Presocratics, and the apparent ability of equally good arguments to support both, for example, Heraclitus (only change occurs) and Parmenides (change does not occur), a loose group known as the "sophists" concluded that philosophy cannot be used to discover truth. And in fact, there is no truth. Man is the measure of all things. Philosophy should be used instead as a tool to convince others of whatever position you desire.
II. Socrates is Ignorant
The Oracle at Delphi was purported to have said that there is no one wiser than Socrates. Socrates took this to mean that he is wise because he recognizes his own ignorance. What he knows is that he knows nothing.
III. The Socratic Method
Thus, seeking knowledge was one of the goals of Socrates. Unlike the Presocratics, who were concerned primarily with speculations about the fundamental nature of reality, Socrates was more interested in ethics. His famous Socratic Method generally starts with a request for a definition of a moral concept (What is courage?), and after the person responds he asks more questions, eventually leading them to a belief that contradicts their original definition. In this way, he shows that they, like Socrates, are not knowledgeable after all.
IV. Socratic Ethics
Like the sophists, Socrates is more concerned with human events than with cosmological theories. However, unlike the sophists he is not a nihilist. He believes that there are moral truths. For Socrates, the goal of human virtue is to seek happiness, but happiness can only come by taking care of one's soul. Virtuous actions improve the soul, and vicious actions harm the soul. This is why doing harm to someone else is worse than receiving harm: the former harms one's soul, whereas the victim only has his body harmed.
Famously, Socrates denies that anyone ever knowingly does evil. Everyone does what he believes to be good for himself, but ignorance of what is genuinely right is what leads people to do evil. If they knew that a virtuous soul leads to happiness, then no one would harm his or her own soul by doing evil. So knowledge leads to morality.