The words that are sung cannot be evicted from a comprehensive image of language, just for the reason of breaking the condition of having an epistemological significance. For any word, even those connected in logical strings of arguments, imposes a specific rhythm, both as regards their persuasive power and their capacity of directing to the truth. Logic has the rhythm of heart beating, which can be easily ignored.
When the words are sung, they express the limits of language, but not the limits between a domain of human knowledge and another, since they do not put the bases of a new kind of science. They are rather the limits put to our expectation that the common discourse cannot impose a rhythm able to move its hearers according to it.
Beneath the problem of truth, the discourses confront the problem of the general habit of resisting to any kind of discourse which does not succeed in imposing its rhythm.