As you may know, Russell wrote "Definition of Number" inspired by Frege's earlier definition. I have heard that he did this for a specific reason, but hours of research have proven to be hopeless. What was the confusion about definition of number that Russell needed to do this? Also, Im not so sure I truelly understand it... "The number of a class is the class of all those classes that are similar to it." thus "A number is anything which is the number of some class." -Bertrand Russell (He is only talking about whole cardinal numbers here of course) I did have a spark somewhere in my brain ( ), but I wanna hear opinions of others as well. What do you think of this definition? How do you understand it?