The biographies of universal geniuses like Leo Battista Alberti or Lionardi [Transcriber's Note: Lionardo] da Vinci, so multiform in their capacity and so creative in their intuitions, prompt us to ask what is the connection between the spirit of an age and the men in whom it is incorporated. Not without reason are we forced to personify the Renaissance as something external to its greatest characters. There is an intellectual strength outside them in the century, a heritage of power prepared for them at birth. The atmosphere in which they breathe is so charged with mental vitality that the least stirring of their special energy brings them into relation with forces mightier than are the property of single natures. In feebler periods of retrospect and criticism we can but wonder at the combination of faculties so varied, and at miracles so easily accomplished. These times of clairvoyance and of intellectual magnetism, when individuals of genius appear to move like vibrios in a life-sustaining fluid specially adapted to their needs, are rare in the history of the world; nor has our science yet arrived at analysing their causes. They are not on that account the less real. To explain them by the hypothesis of a _Weltgeist_, the collective spirit of humanity proceeding in its evolution through successive phases, and making its advance from stage to stage by alternations of energy and repose, is simply to restore, in other terms, a mystery that finds its final and efficient cause in God.[2] [Footnote 2: The analogy of the individual might be quoted. We are aware within ourselves of times when thought is fertile and insight clear, times of conception and projection, followed by seasons of slow digestion, assimilation, and formation, when the creative faculty stagnates, and the whole force of the intellect is absorbed in mastering through years what it took minutes to divine.]