Most researchers believe that Friedrich Nietzsche (1844–1900) knew little of the 19th century philosopher Søren Kierkegaard (1813–1855). Georg Brandes, a Danish philosopher, wrote to Nietzsche in 1888 asking him to study the works of Kierkegaard, to which Nietzsche replied that he would. Nietzsche seems to have been unable to undertake this task before his mental collapse in 1889.