A fugue is the epitome
music, which is itself epitomized in the High Baroque
of J.S. Bach
The particular choice of the theme is considered to be the genius of Bach: a scrap of music
sufficiently ambiguous to suffer transformation in any number of ways, and played back upon itself.
The theme may be transposed up or down a certain number of steps, say a fourth or a fifth; it may be inverted, and played upside down; its time may be speeded up or slowed down; it may be played upon itself with a varying delay in starting.
Practically, there are more than a few problems; one of the most straightforward is: how to play, say, four voices, independant, and altered in the ways mentioned above, when people have only two hands. The solution, simply put, is for each hand to be several voices, and to divide voices between them.
To think about these parts of the fugue is easier when one thinks of a choir with several voices. I always tell my students to play each part separately, regardless of how it will eventually be played, until they can hear each voice clearly, distinctly, separately.
You can't play the voice unless you can hear it; and you can't hear it unless you can play it. For all this sounds like a paradox, it is readily cut by practice.
More interestingly, there are the implications regarding the consciousness that could compose such things, and perform them. Suggestions have been made concerning the, possible, multi-furcations (to coin a phrase) of the baroque mind, being able to play these voices against themselves, like a choir in the head, so to speak.
I, however, hold to the view that it is the polyphonic method itself that is the genius. Rather like some kind of machine developed conceptually by the composers of the era, that, in the hands of the greatest practioners, produced what Lord Brawl calls ominous music, what I prefer to call celestial music.
In some ways, it is only in the choice of the theme that the composer inserts himself into the process.
It is the performer, it seems to this humble piano teacher, that has the harder task, beyond the conceptual multi-furcation: he must, like Glenn Gould, multi-furcate his very hands, and fingers--a process physical, as well as mental.
And if he must sing, unintentionally, but seemingly necessarily, who can condem him.