ings and simple repetitions. This was certainly known by Aristotle,
as he clearly shows in both the Rhetoric and the Poetics, and has
also figured largely in the thought of recent psychologists of per-
ception such as Gombrich and Arnheim. ⁴ In terms of poetry and
music, however, this tendency to form symmetrical units is not
linked to the notion of static structures so much as it is linked to
rhythm. Rhythm is a concept common to both music and poetry,
but one that is extremely difficult to define. Most definitions, in dic-
tionaries or in specialized reference books dealing with music or
poetry, tend to describe rhythm as the symmetrical repetition of
measurable units in time made up of both strong and weak ele-
ments, and with a strong degree of differentiation between the ele-
ments. ⁵ Symmetrical repetition facilitates the measurement of
rhythm, but it is not a prerequisite for rhythm. Rhythm can also be
asymmetrical in nature. For our purposes, rhythm is the sequential
unfolding of discrete units in time. Symmetrical repetition is
merely the most common rhythmic phenomenon.

The tendency to form symmetrical phrases in Western music
is highly similar to the ancient and ubiquitous practice, in literary
discourse, of organizing language in syntactical units of uniform
length. Roland Barthes, who published an outline of rhetoric in
Communications, credits Aristotle with designating a phrase that
has a beginning and an end ("début et fin"), with at least two mem-
bers ("élévation et abaissement") and at most four members, as a
period. ⁶ A period for Aristotle is, in effect, a microcosm of his
definition of a proper plot for a tragedy. Like a plot, it must have a
beginning, an end, and a length that one can grasp easily. Like the
plot, the period must be a complete, self-contained entity. Aristotle
defines the period succinctly in the Rhetoric:

By a period I mean a portion of speech that has in itself a beginning and
an end, being at the same time not too big to be taken in at a glance. Lan-
guage of this kind is satisfying and easy to follow. It is satisfying, because
it is just the reverse of indefinite; and moreover, the hearer always feels
that he is grasping something and has reached some definite conclusion;
whereas it is unsatisfactory to see nothing in front of you and get no-
where. It is easy to follow, because it can easily be remembered; and this
because language when in periodic form can be numbered, and number
is the easiest of all things to remember. That is why verse, which is mea-
sured, is always more easily remembered than prose, which is not . . . ⁷

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