Pyotr or Petr Yakovlevich Chaadayev (; also spelled Chaadaev; 7 June [27 May O.S.] 1794 – 26 April [14 April O.S.] 1856) was a Russian philosopher. He was one of the Russian Schellingians.
Chaadayev was born in Moscow into a wealthy noble family. He interrupted his education to join the military and served with distinction in the Napoleonic Wars. Chaadayev wrote eight "Philosophical Letters" His maternal grandfather was Prince Mikhail Shcherbatov, a well-known intellectual who wrote A Discourse on the Corruption of Morals in Russia. After the early deaths of his mother and father, he was brought up by his uncles and aunts. He was educated by French and German governesses and teachers and became totally fluent in French early in his life; he was also able to read well in German.
When in 1836 the first edition (and only one published during his life) Chaadayev's case may have been the first recorded incident in Russia in which psychiatry was used to suppress dissent.
Living under house arrest following his declaration of insanity, Chaadayev's next work was entitled, fittingly, "Apologie d'un Fou" (which has been translated as "Apology of a Madman" but may better be translated as "Apologia of a Madman") (1837). It opens with a quote from Samuel Coleridge stating "O my brethren! I have told / Most bitter truth, but without bitterness." In this brilliant but uncompleted work he maintained that Russia must follow her inner lines of development if she was to be true to her historical mission.
His ideas influenced both the Westernizers (who supported bringing Russia into accord with developments in Europe by way of various degrees of liberal reform) and Slavophiles (who supported Russian Orthodoxy and national culture).
According to Chaadayev, Russian history had not developed normally and lacked intellectual and cultural continuity. This was because Russians never had a "great, universal spiritual ideal" to motivate them. He saw Peter the Great as the first leader to attempt to impart historical consciousness to the Russian people by connecting them with European history. In his view, Russian leaders should follow Peter's example and adopt only the beneficial parts of European culture. He saw this is a step towards his ultimate goal: the establishment of "the kingdom of God on earth, the intellectual, moral, and cultural unification of mankind."