In the 17th century, in the "Meditations on First Philosophy, in which the Existence of God and the Immortality of the Soul are Demonstrated" (1641), René Descartes said that the standard of truth is self-evidence of clear and distinct ideas. Despite the logician Descartes' understanding of "self-evident truth", the philosopher Descartes considered that the self-evident truth of "two plus two equals four" might not exist beyond the human mind; that there might not exist correspondence between abstract ideas and concrete reality.