I have written a polynomial class along the lines described in SICP 2.5.3 (except using defclass). I would like to be able to seamlessly add and multiply polynomials and regular numbers but I can't make change-class accept a number. I tried to simplify the problem by changing class from an integer to a float: (change-class 4 'float) but that gave me the error: There is no applicable method for the generic function #<STANDARD-GENERIC-FUNCTION CHANGE-CLASS (7)> when called with arguments (4 #<BUILT-IN-CLASS FLOAT>). [Condition of type SIMPLE-ERROR] I get an error of the same form from (fyi): (change-class 4 'polynomial) I'm going to go ahead and implement a manual conversion but I would prefer to use the built-in clos facilities.