Aporia (Greek: : impasse; lack of resources; puzzlement) denotes, in philosophy, a philosophical puzzle or state of puzzlement, and, in rhetoric, a rhetorically useful expression of doubt.
Plato's early dialogues are often called his 'aporetic' dialogues because they typically end in aporia. In such a dialogue, Socrates questions his interlocutor about the nature or definition of a concept, for example virtue or courage. Socrates then, through elenctic testing, shows his interlocutor that his answer is unsatisfactory. After a number of such failed attempts, the intelocutor admits he is in aporia about the examined concept, that he does not know what it is. In Plato's Meno (84), Socrates describes the purgative effect of reducing someone to aporia: it shows someone who merely thought he knew something that he does not in fact know it and instils in him a desire to investigate it.
In Aristotle's Metaphysics aporia plays a role in his method of inquiry. In contrast to a rationalist inquiry that begins from a priori principles, or an empiricist inquiry that begins from a tabula rasa, Aristotle begins his inquiry in the Metaphysics by surveying the various aporiai that exist, drawing in particular on what puzzled his predecessors. Aristotle claims that 'with a view to the science we are seeking (i.e. metaphysics), it is necessary that we should first review the things about which we need, from the outset, to be puzzled' (994a). Book Beta of the Metaphysics is a list of the aporiai that preoccupy the rest of the work.