1860 his health began to decline, and he experienced shortness of breath and heart palpitations; in September he suffered inflammation of the lungs and, although he was starting to recover, he remained very weak. The last friend to visit him was Wilhelm Gwinner; according to him, Schopenhauer was concerned that he would not be able to finish his planned additions to Parerga and Paralipomena but was at peace with dying. He died of pulmonary-respiratory failure on 21 September 1860 while sitting at home on his couch. He died at the age of 72 and had a funeral conducted by a Lutheran minister. Philosophy The world as representation Schopenhauer saw his philosophy as an extension of Kant's, and used the results of Kantian epistemological investigation (transcendental idealism) as starting point for his own. Kant had argued that the empirical world is merely a complex of appearances whose existence and connection occur only in our mental representations. Schopenhauer did not deny that the external world existed empirically but followed Kant in claiming that our knowledge and experience of the world is always indirect. Schopenhauer reiterates this in the first sentence of his main work: "The world is my representation (Die Welt ist meine Vorstellung)". Everything that there is for cognition (the entire world) exists simply as an object in relation to a subject—a 'representation' to a subject. Everything that belongs to the world is, therefore, 'subject-dependent'. In Book One of The World as Will and Representation, Schopenhauer considers the world from this angle—that is, insofar as it is representation. Theory of perception In November 1813 Goethe invited Schopenhauer to help him on his Theory of Colours. Although Schopenhauer considered colour theory a minor matter, he accepted the invitation out of admiration for Goethe. Nevertheless, these investigations led him to his most important discovery in epistemology: finding a demonstration for the a priori nature of causality. Kant openly admitted that it was Hume's skeptical assault on causality that motivated the critical investigations in Critique of Pure Reason and gave an elaborate proof to show that causality is a priori. After G. E. Schulze had made it plausible that Kant had not disproven Hume's skepticism, it was up to those loyal to Kant's project to prove this important matter. The difference between the approaches of Kant and Schopenhauer was this: Kant simply declared that the empirical content of perception is "given" to us from