According to pickaxe2, p516:
The Kernel module is included by class Object, so its [instance] methods
are available in every Ruby object. The Kernel instance methods are
documented in class Object beginning on page 567. This section
documents the module methods. These methods are called without a
receiver and thus can be called in functional form.
I tried to model that state of affairs with this code:
r2test.rb:16: undefined local variable or method `test1’ for O:Class
How come you can call Kernel methods without a receiver?