Plato's Socratic dialog Euthyphro is in many ways archetypal of the sort of philosophy that Socrates is thought to have been interested in. In it (as in most classic 'Socratic dialogs'), Socrates seeks out a person who claims to have a certain sort of knowledge. He then proceeds to show that these experts do not possess this knowledge by getting them to contradict themselves. With this in mind, I will discuss the three definitions of the word 'pious' that the character Euthyphro gives to Socrates, and Socrates' problems with each of these definitions.
The dialog begins with Socrates and Euthyphro meeting at the king-archon's court; Socrates has been summoned with charges of corrupting the youth and impiety, and Euthyphro wishes to prosecute his father for leaving one of his servants, a murderer, to die. It is at this moment that Socrates first asks Euthyphro, a young priest who considers himself an expert on piety, what is pious. Socrates claims that if he can convince the court that he has learned the meaning of piety from Euthyphro, they might dismiss the charges against him. It is clear from the start that a lexical definition of piety is not what Socrates is interested in. Rather, he is looking for a practical definition of the pious. A definition of Socrates approval would allow him to look at any action and determine as to whether or not it is pious.
Euthyphro's first definition of piety reads, “I say that the pious is to do what I am doing now, to prosecute the wrongdoer, be it about murder or temple robbery or anything else, whether the wrongdoer is your father or your mother or anyone else...” (5E) This initial definition is rather specific, and claims that piety is just to prosecute or punish people when they commit some sort of crime or immoral act.
Socrates too thinks that this definition is too narrow. He asks Euthyphro if there are other actions which count as pious, to which Euthyphro admits that there are. (6D) If there are actions other than punishing wrongdoers that count as pious, then Euthyphro's initial definition is consequently rendered invalid. A definition of a word ought describe all instances of that word, and Euthyphro has just admitted that there are actions outside of his definition which also count as pious. It might be that it is pious to prosecute wrongdoers, but this in not an exhaustive description of all pious actions.
Socrates presses Euthyphro for another definition of piety, begging that he answer the question properly. Euthyphro replies, “...what is dear to the god's is pious, what is not is impious.” (7A) This is a much different definition than the first—it has a much greater scope. Euthyphro argues that the gods have some ability such that their love of something makes it pious. Similarly, the gods have some power such that what they disapprove of or simply do not like is consequently impious. This is precisely the sort of definition that Socrates is looking for; if it is true, then all...