This chapter defines paradoxes. It reviews several definitions, demonstrating the difference between contradictions and paradoxes. The essence of paradoxes is that they deliver a certain truth and a higher-level meaning. Contradictions are conflicting elements within the same system, whereas paradoxes are conflicting elements that reveal a previously unknown truth. A definition derived from the field of psychotherapy is also mentioned: Paradoxes are best characterized as unacceptable conclusions derived by apparently acceptable reasoning from apparently acceptable premises. Paradoxes are also seen as unacceptable conclusions derived from apparently acceptable reasoning based on seemingly acceptable premises. The definition proposed for this book is “a statement that is seemingly contradictory or opposed to common sense and yet is perhaps true,” including the broader notion of an “air of absurdity,” provided that this absurdity carries a higher-level meaning. Some historical examples are presented, such as Achilles never catching up with a much slower tortoise, the arrow paradox, the paradox of place, the liar paradox, identity paradox, and the paradox of the stone.