Baruch Spinoza, though he argued for the existence of a permanent reality, asserts that all phenomenal existence is transitory. In his opinion sorrow is conquered "by finding an object of knowledge which is not transient, not ephemeral, but is immutable, permanent, everlasting." The Buddha taught that the only thing which is eternal is Nirvana. David Hume, after a relentless analysis of the mind, concluded that consciousness consists of fleeting mental states. Hume's Bundle theory is a very similar concept to the Buddhist "skandhas", though his skepticism about causation lead him to opposite conclusions in other areas. Arthur Schopenhauer's philosophy parallels Buddhism in his affirmation of asceticism and renunciation as a response to suffering and desire (cf. Schopenhauer's "The World as Will and Representation", 1818).