Only through the use of koans. Consider the dialog in:

http://en.wikipedia.org/wiki/What_the_Tortoise_Said_to_Achilles

Could you explain what "If A, then B" means, tabooing "if/then","therefore",etc.?

Here is another way:

If a rational agent becomes aware that the statement "I should do X" is true, then it will either proceed to do X or proceed to realize that it cannot do X (at least for now).

ETA: Here is a simple Python function (I think I coded it correctly):

def square (x): y=x*x return y

"return" is not just another symbol. It is not a gensym. It is functional. The act of returning and producing an output is completely separate from and non-reducible-to everything else that a subroutine can do.

Rational agents use "should" the same way this subroutine uses "return". It controls their output.