Initially designating a lesser evil, tolerance relieves itself of its negative charge to gain a positive dimension, acquired at the time of the crise de la conscience européenne, of which the men of the 18th century are heirs. Like happiness, it is one of the key words of the Enlightenment which encyclopedias do not fail to seize. However, the success of this notion does not overshadow the ambiguity it carries within it. Reading the entry “Tolerance” of the three major encyclopaedias of the time, those of Diderot and d'Alembert, De Felice and Panckoucke, testifies to this plural understanding. It appears sometimes as an essential virtue that the statesman must hasten to adopt, sometimes as a dangerous principle with subversive potential that the sovereign must beware of.