The Divine Comedy can be described simply as an allegory: each canto, and the episodes therein, can contain many alternative meanings. Dante's allegory, however, is more complex, and, in explaining how to read the poem – see the Letter to Cangrande – he outlines other levels of meaning besides the allegory: the historical, the moral, the literal, and the anagogical.
The structure of the poem, likewise, is quite complex, with mathematical and numerological patterns arching throughout the work, particularly threes and nines, which are related to the Trinity. The poem is often lauded for its particularly human qualities: Dante's skillful delineation of the characters he encounters in Hell, Purgatory, and Paradise; his bitter denunciations of Florentine and Italian politics; and his powerful poetic imagination. Dante's use of real characters, according to Dorothy Sayers in her introduction to her translation of the Inferno, allows Dante the freedom of not having to involve the reader in description, and allows him to "[make] room in his poem for the discussion of a great many subjects of the utmost importance, thus widening its range and increasing its variety."
Dante called the poem "Comedy" (the adjective "Divine" was added later in the 16th century) because poems in the ancient world were classified as High ("Tragedy") or Low ("Comedy"). Low poems had happy endings and were written in everyday language, whereas High poems treated more serious matters and were written in an elevated style. Dante was one of the first in the Middle Ages to write of a serious subject, the Redemption of humanity, in the low and "vulgar" Italian language and not the Latin one might expect for such a serious topic. Boccaccio's account that an early version of the poem was begun by Dante in Latin is still controversial.
Although the Divine Comedy is primarily a religious poem, discussing sin, virtue, and theology, Dante also discusses several elements of the science of his day (this mixture of science with poetry has received both praise and blame over the centuries). The Purgatorio repeatedly refers to the implications of a spherical Earth, such as the different stars visible in the southern hemisphere, the altered position of the sun, and the various timezones of the Earth. For example, at sunset in Purgatory it is midnight at the Ebro, dawn in Jerusalem, and noon on the River Ganges:
Just as, there where its Maker shed His blood,
the sun shed its first rays, and Ebro lay
beneath high Libra, and the ninth hour's rays
were scorching Ganges' waves; so here, the sun
stood at the point of day's departure when
God's angel—happy—showed himself to us.
Dante travels through the centre of the Earth in the Inferno, and comments on the resulting change in the direction of gravity in Canto XXXIV (lines 76–120). A little earlier (XXXIII, 102–105), he queries the existence of wind in the frozen inner circle of hell, since it has no temperature differentials.
Inevitably, given its setting, the Paradiso discusses astronomy extensively, but in the Ptolemaic sense. The Paradiso also discusses the importance of the experimental method in science, with a detailed example in lines 94–105 of Canto II:
Yet an experiment, were you to try it,
could free you from your cavil and the source
of your arts' course springs from experiment.
Taking three mirrors, place a pair of them
at equal distance from you; set the third
midway between those two, but farther back.
Then, turning toward them, at your back have placed
a light that kindles those three mirrors and
returns to you, reflected by them all.
Although the image in the farthest glass
will be of lesser size, there you will see
that it must match the brightness of the rest.