We tend to believe the premises because we can see that their consequences are true, instead of believing the consequences because we know the premises to be true. But the inferring of premises from consequences is the essence of induction; thus the method in investigating the principles of mathematics is really an inductive method, and is substantially the same as the method of discovering general laws in any other science.
"The Regressive Method of Discovering the Premises of Mathematics" (1907), in Essays in Analysis (1973), pp. 273–274