A phenomenon (Greek: φαινόμενον, phainómenon, from the verb φαίνειν, phainein, to show, shine, appear, to be manifest or manifest itself, plural phenomena) is "an observable fact or event". The term came into its modern philosophical usage through Immanuel Kant, who contrasted it with the noumenon. In contrast to a phenomenon, a noumenon cannot be directly observed. Kant was heavily influenced by Gottfried Wilhelm Leibniz in this part of his philosophy, in which phenomenon and noumenon serve as interrelated technical terms. Far predating this, the ancient Greek Pyrrhonist philosopher Sextus Empiricus also used phenomenon and noumenon as interrelated technical terms.