Theaetetus

Aporia is Greek term meaning puzzlement or perplexation. It is the goal of Socratic dialogue, to lead the individual to a state of doubt regarding previously established truths and wisdom.

A priori is commonly contrasted with a posteriori. While a posteriori knowledge is “based solely on experience or personal observation,” a priori knowledge stems from “reasoning based on self-evident truths […] for example, ‘Every mother has had a child.’” (“A priori.” Merriam-Webster.com Dictionary, Merriam-Webster, www.merriam-webster.com/dictionary/a%20priori.)

In the context of Theaetetus, being is equivalent to the essence or essential nature of something.