So the freaky thing about metaprogramming is that the model changes itself right? The aphorism in three simple sentences:

1 "data must express procedure"

2 "text must express procedure"

3 "text must express data"

This shows that text both directly expresses procedure by rule 2, and indirectly by rule 1 via rule 3. This indirection step allows recursion to take place (i.e. the data contains a model in the same language as it is defined in) that is used for the model to reason about itself.