Button isEnabled method requiring a boolean responds unexpectedly

T works, True does not
F works, and so does False and Nil

Why not True?

It depends what True is set to. You could (setq True 'T), then it will work, otherwise the symbol True will be NIL.

Since T is true by lisp and True is just an undefined symbol evaluating to nil unless otherwise set.

Any an symbol evaluating to nil would serve as false.

Best to use t and nil.

Thanks,
Jim