Francis North, in his Philosophical Essay of Musick, makes a distinction between two types of contrapuntal writing, each of them described from two different vantage points—that of the listener and that of the composer. The first type of contrapuntal writing, consisting of fugues and points of imitation, is “extream delightfull”: it can be followed by the listener (“carrying the attendance of the mind from one part to another”), and poses no particular technical challenge to the composer (they “restrain not the fancy of the Composer, but advance it”). North's reference to the composer's task is somewhat unclear—it is hard to determine if by the phrase “restrain not the fancy of the Composer” North meant that fugues and points of imitation actually encouraged composers’ fancy by forcing them to solve self-imposed contrapuntal puzzles (triple invertible counterpoint, various interlocks), or, alternatively, North simply overlooked those considerable intellectual challenges practiced by several significant composers active at the time.

The second type of contrapuntal writing, of which North mentions only “Descant upon plainsong Cannons” explicitly, involves more elaborate “other tricks.” North finds this category useful only for purposes of teaching and demonstration of skill. North's comment on the listener's understanding of such elaborate contrapuntal writing is telling: “[…] such fetter’d Musick is alwayes worse, and the hearer shall not observe the Art till he be told of it: it is like Acrostick Verse, or rhithm in Plays, which shew skill or labour, but serve not those ends for which the pieces were designed.”

It is hard to say if North meant that the listener, after being told of “the Art,” would have been able to follow and appreciate the art. To “observe the Art” (that is, to observe the technical aspects of the music) might mean merely to acknowledge it, probably even understand it, but not necessarily be able to imitate it. In the following chapter I will refer to these compositional tricks as “indiscernible” structures—thereby making a sweeping generalization regarding listeners’ perception, despite the fact that during subsequent listening, the listener may already acknowledge the “Art,” and that attentive skilled musicians may be able to realize while listening that something in the composition merits another look at the score.