faculty of reasoning, but the faculty of discovering.  When a man possesses it to a certain degree, we call him a man of genius.  Genius, or the faculty of discovering, is the generating principle of science.  Still, strange to say, this principle is scarcely pointed out by a great number of logicians.  They develop at length the rules of observation and the rules of reasoning; and it seems that, in their idea, the conjunction of reason and experience is effected all alone and of necessity.  I taught logic myself in this way for twenty years, until one day, thinking better upon the subject, I was obliged to say to myself (forgive me this rather trivial quotation): 

     Tu n’avais oublie qu’un point: 
     C’etait d’eclairer ta lanterne.[163]

The meeting together of the understanding and of facts is a discovery; and discovery depends upon a faculty sung by poets, admired by mankind, and too little noticed by logicians—­genius.  Genius has for its characteristic a sudden illumination of the mind, a gratuitous gift and one which cannot be purchased.  But let us hasten to supply a necessary explanation.  Genius is a primitive fact, a gift; but the work of genius has conditions, or rather a condition—­labor.  Labor does not replace genius, but genius does not dispense with labor; nature only delivers up her secrets to those who observe her with long patience.  Newton was asked one day how he had found out the system of the universe.  He replied with a sublime naivete:  “By thinking continually about it.”  He so pointed out the condition of every great discovery; but he forgot the cause—­the peculiar nature of his own intellect.  It was necessary to be always pondering the motions of the stars; but it was necessary moreover to be Isaac Newton.  So many had thought on the subject, as long perhaps as he, and had not made the discovery.

Labor, the condition of discoveries, should have as its effect to recognize the methods really appropriate to the nature of the inquiries, and to keep the mind well informed in existing science.  In fact, every scientific discovery supposes a series of previous discoveries which have brought the mind to the point at which it is possible to see something new.  For this reason it is that a discovery often presents itself to two or three minds at once, when there are found, at the same epoch, two or three minds endowed with the same power.  They see all together because the onward progress of science has brought them to the same summit:  this is the condition; and because they have the same power of vision:  this is the cause.  There is therefore a method for putting ourselves on the road to discovery, but no method for making the discovery itself.  The man of genius sees where others do not see; and when he has seen, everybody sees after him.  If, furnished with Gyges’ ring, you could gain access to the studies of savants at the moment when a great discovery has just been made, you would see more than one of them striking his forehead and exclaiming:  “Fool that I was! how could I help seeing it? it was so simple.”  Truth appears simple when it has been discovered.