Mark Pattison, Memoirs of an Oxford Don (London: Cassell, 1988; 1885), pp. 51-52:
A man who does not know what has been thought by those who have gone before him is sure to set an undue value upon his own ideas – ideas which have perhaps been tried and found wanting. As accumulated learning stifles the mental powers, so original thinking has been known to bring about a puffy, unsubstantial mental condition.