How is it that so many philosophers both ancient and modern have so long been discussing psychological questions without having arrived at the truth?

"Those men were precursors of the eternal truths of the true spiritist doctrine, for which they have prepared the way. They were men, and therefore subject to error, because they often mistook their own ideas for the true light; but their very errors have served the cause of truth by bringing into relief both sides of the argument. Moreover, among those errors are to be found many great truths which a comparative study of the various theories thus put forth would enable you to discover." - Allan Kardec, The Spirits Book, 1857 http://ssbaltimore.org/e-books/