Ostensibly in order to better defend himself in an upcoming trial for being an impious citizen of Athens, Socrates asks Euthyphro for a clear definition of piety (holiness); he offers Socrates four definitions.
Euthyphro's first definition of piety is what he is doing now, that is, prosecuting his father for manslaughter (5d). Socrates rejects Euthyphro's action, because it is not a definition of piety, and is only an example of piety, and does not provide the essential characteristic that makes pious actions pious.
Euthyphro's second definition: Piety is what is pleasing to the gods. (6e–7a) Socrates applauds this definition, because it is expressed in a general form, but criticizes it saying that the gods disagree among themselves as to what is pleasing. This means that a given action, disputed by the gods, would be both pious and impious at the same time – a logical impossibility. Euthyphro argues against Socrates' criticism, by noting that not even the gods would disagree, among themselves, that someone who kills without justification should be punished. Yet Socrates argues that disputes would still arise – over just how much justification actually existed; hence, the same action could be pious and impious; again, Euthyphro's definition cannot be a definition of "piety".
To overcome Socrates' objection to his second definition of piety, Euthyphro amends his definition. (9e)
Euthyphro's third definition of piety is: "What all the gods love is pious, and what they all hate is impious." In reply, Socrates poses the question that would eventually become known in philosophy as the Euthyphro dilemma: "Is the pious loved by the gods because it is pious? Or is it pious because it is loved by the gods?". Euthyphro seems unsure as to what the question means and so Socrates applies a dialectic technique: an analogy, to clarify his question (10a). He persuades Euthyphro to agree that when we call a thing "carried", it simply because it is being carried by someone and not because it possesses an inherent characteristic, which could be called "carried". That is, "being carried" is not an essential trait of the thing being carried but a condition, a state of that the object is currently in. He then moves to what we call "beloved" (φιλούμενόν filoumenon). Is something "beloved" in and of itself (like being big or red), or does it become beloved when it is loved by someone? Clearly, the answer is again the latter, something becomes beloved when it is loved. So then, continues Socrates, something beloved by the gods (θεοφιλές theofiles) becomes so because it is loved by them, to which Euthyphro agrees and Socrates moves to the conclusion that reveals his contradiction: What is beloved by the gods cannot be pious. Euthyphro seems to be taken aback so Socrates reminds him the definitions he gave previously (10e). He had said that something is loved by the gods because it is pious, which means that their love follows from something inherent in the pious. And yet they just agreed that what is beloved is put in that state as a result of being loved. So piety cannot belong to what is beloved by the gods since according to Euthyphro it does not acquire its characteristics by something (the act of being loved) by has then a priori, in contrast to the things that are beloved that are put in this state through the very act of being loved. It seems therefore that Euthyphro's third argument is flawed.
At that juncture of their dialogue, Euthryphro does not understand what makes his definition of "piety" a circular argument; he agrees with Socrates that the gods like an action because it is pious. Socrates then argues that the unanimous approval of the gods is merely an attribute of "piety", that divine approval is not a defining characteristic of "piety". That divine approval does not define the essence of "piety", does not define what is "piety", does not give an idea of "piety"; therefore, divine approval is not a universal definition of "piety".
- Linguistic note
Socrates argument is convoluted not only because of its structure but because of the language used, and is said to have "reduced translators to babble and driven commentators to despair". The text presents the argument through a distinction between the active and the passive voice, as for example when Socrates asks about the difference between a "carried thing" (φερόμενον) and "being carried" (φέρεται), both using the word "carried" in the English translation.
In the second half of the dialogue, Socrates suggests a definition of "piety", which is that "piety is a species of the genus 'justice'" (12d), but he leads up to that definition with observations and questions about the difference between species and genus, starting with the question:
... Are you not compelled to think that all that is pious is just?
Yet, Socrates later says that the information provided in his question to Euthyphro is insufficient for a clear definition of "piety", because piety belongs to those actions we call just, that is, morally good; however, there are actions, other than pious actions, which we call just (12d); for example, bravery and concern for others. Socrates asks: What is it that makes piety different from other actions that we call just? We cannot say something is true, because we believe it to be true. We must find proof.