Rational animal is a classical definition of man. Though it is often attributed to first appearing as a definition in Aristotle

Aristotle

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

's Metaphysics

Metaphysics (Aristotle)

Metaphysics is one of the principal works of Aristotle and the first major work of the branch of philosophy with the same name. The principal subject is "being qua being", or being understood as being. It examines what can be asserted about anything that exists just because of its existence and...

, Aristotle does not define it here. In the Nicomachean Ethics

Nicomachean Ethics

The Nicomachean Ethics is the name normally given to Aristotle's best known work on ethics. The English version of the title derives from Greek Ἠθικὰ Νικομάχεια, transliterated Ethika Nikomacheia, which is sometimes also given in the genitive form as Ἠθικῶν Νικομαχείων, Ethikōn Nikomacheiōn...

 I.13, Aristotle states that the human being has a rational principle.

Descartes

In Meditation II of Meditations on First Philosophy

Meditations on First Philosophy

Meditations on First Philosophy is a philosophical treatise written by René Descartes and first published in 1641 . The French translation was published in 1647 as Méditations Metaphysiques...

, Descartes arrives at his famous "I am, I exist"

Cogito ergo sum

is a philosophical Latin statement proposed by . The simple meaning of the phrase is that someone wondering whether or not they exist is, in and of itself, proof that something, an "I", exists to do the thinking — However this "I" is not the more or less permanent person we call "I"...

 claim. He then goes on to wonder "What am I?" He considers and rejects, "rational animal":

Shall I say 'a rational animal'? No; for then I should have to inquire what an animal is, what rationality is, and in this one question would lead me down the slope to other harder ones.

Quotes

• Man is a rational animal who always loses his temper when called upon to act in accordance with the dictates of reason. – Oscar Wilde
  Oscar Wilde
  Oscar Fingal O'Flahertie Wills Wilde was an Irish writer and poet. After writing in different forms throughout the 1880s, he became one of London's most popular playwrights in the early 1890s...
  
  
• It has been said that man is a rational animal. All my life I have been searching for evidence which could support this. – Bertrand Russell
  Bertrand Russell
  Bertrand Arthur William Russell, 3rd Earl Russell, OM, FRS was a British philosopher, logician, mathematician, historian, and social critic. At various points in his life he considered himself a liberal, a socialist, and a pacifist, but he also admitted that he had never been any of these things...