After an initial ascension, Beatrice guides Dante through the nine celestial spheres of Heaven. These are concentric and spherical, as in Aristotelian and Ptolemaic cosmology. While the structures of the Inferno and Purgatorio were based on different classifications of sin, the structure of the Paradiso is based on the four cardinal virtues and the three theological virtues.

The first seven spheres of Heaven deal solely with the cardinal virtues of Prudence, Fortitude, Justice and Temperance. The first three describe a deficiency of one of the cardinal virtues - the Moon, containing the inconstant whose vows to God waned as the moon thus lack fortitude; Mercury, containing the ambitious who were virtuous for glory and thus lacked justice; and Venus, containing the lovers, whose love was directed toward another than God and thus lacked Temperance. The final four incidentally are positive examples of the cardinal virtues, all led on by the Sun, containing the prudent, whose wisdom lighted the way for the other virtues, to which the others are bound (constituting a category on its own). Mars contains the men of fortitude who died in the cause of Christianity; Jupiter contains the kings of Justice; and Saturn contains the temperant, the monks who abided to the contemplative lifestyle. The seven subdivided into three are raised further by two more categories: the eighth sphere of the fixed stars that contain those who achieved the theological virtues of faith, hope and love, and represent the Church Triumphant - the total perfection of man, cleansed of all the sins and carrying all the virtues of heaven; and the ninth circle, or Primum Mobile (corresponding to Medieval astronomy of Geocentricism) which contains the angels, creatures never poisoned by original sin. Topping them all is the Empyrean that contains the essence of God, completing the 9 fold division to 10.

Dante meets and converses with several great saints of the Church, including St Thomas Aquinas, St Bonaventure, St Peter, and St John. The Paradiso is consequently more theological in nature than the Inferno and the Purgatorio. However, Dante admits the vision of heaven he receives is the one that his human eyes permit him to see, and the vision of heaven found in the Cantos is Dante's own personal one.

The Divine Comedy finishes with Dante seeing the Triune God. In a flash of understanding, which he cannot express, Dante finally understands the mystery of Christ's divinity and humanity, and his soul becomes aligned with God's love:

But already my desire and my will
were being turned like a wheel, all at one speed,
by the Love which moves the sun and the other stars.

Earliest manuscripts

Thematic concerns

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 14th century) because poems in the ancient world were classified as High ("Tragedy") or Low ("Comedy"). Low poems had happy endings and were of everyday or vulgar subjects, while High poems were for more serious matters. Dante was one of the first in the Middle Ages to write of a serious subject, the Redemption of Man, in the low and vulgar Italian language and not the Latin language as 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.

Dante's personal involvement

In his allegorical description of sin (in the Inferno) and virtue (in the Purgatorio Paradiso), Dante draws on real characters from ancient Greek and Roman myths and history, and from his own times. However, his own actions often also illustrate the concepts he is discussing. For example, Dante shares the fleshly sins of the damned at several points in the upper circles of Hell. At the first circle where the virtuous pagans who pursued honor above all else are punished by eternally knowing they have fallen short for their lack of faith, Dante shares with them their love of honor, as evidenced by the word "honor" being used repeatedly in the Canto. Similarly, at the third circle where Ciacco and other gluttons are punished for their appetites, Dante's appetite for political information about his fellow Florentines appears equally gluttonous:

And I to him: I wish thee still to teach me,
And make a gift to me of further speech.

Farinata and Tegghiaio, once so worthy,
Jacopo Rusticucci, Arrigo, and Mosca,
And others who on good deeds set their thoughts,

Say where they are, and cause that I may know them;
For great desire constraineth me to learn
If Heaven doth sweeten them, or Hell envenom.

Conversely, in the Purgatorio, after leaving the terrace of the proud, Dante has learned from the example set by Omberto and suppresses his own pride, declining to speak of his achievements:

And I: Through midst of Tuscany there wanders
A streamlet that is born in Falterona,
And not a hundred miles of course suffice it;

From thereupon do I this body bring.
To tell you who I am were speech in vain,
Because my name as yet makes no great noise."

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 (a river in Spain), 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.

A briefer example occurs in Canto XV of the Purgatorio (lines 16 – 21), where Dante points out that both theory and experiment confirm that the angle of incidence is equal to the angle of reflection. Other references to science in the Paradiso include descriptions of clockwork in Canto XXIV (lines 13 – 18), and Thales' theorem about triangles in Canto XIII (lines 101 – 102).

Galileo Galilei is known to have lectured on the Inferno, and it has been suggested that the poem may have influenced some of Galileo's own ideas regarding mechanics.

Islamic philosophy

In 1919, Professor Miguel Asín Palacios, a Spanish scholar and a Catholic priest, published La Escatología musulmana en la Divina Comedia ("Islamic Eschatology in the Divine Comedy"), an account of parallels between early Islamic philosophy and the Divine Comedy. Palacios argued that Dante derived many features of and episodes about the hereafter from the spiritual writings of Ibn Arabi and from the Isra and Mi'raj or night journey of Muhammad to heaven. The latter is described in the Hadith and the Kitab al Miraj (translated into Latin in 1264 or shortly before as Liber Scalae Machometi, "The Book of Muhammad's Ladder"), and has some slight similarities to the Paradiso, such as a sevenfold division of Paradise, although this is not unique to the Kitab al Miraj.

Some "superficial similarities" of the Divine Comedy to the Resalat Al-Ghufran or Epistle of Forgiveness of Al-Ma'arri have also been mentioned in this debate. The Resalat Al-Ghufran describes the journey of the poet in the realms of the afterlife and includes dialogue with people in Heaven and Hell, although, unlike the Kitab al Miraj, there is little description of these locations, and it is unlikely that Dante borrowed from this work.

Dante did, however, live in a Europe of substantial literary and philosophical contact with the Muslim world, encouraged by such factors as Averroism and the patronage of Alfonso X of Castile. Of the twelve wise men Dante meets in Canto X of the Paradiso, Thomas Aquinas and, even more so, Siger of Brabant were strongly influenced by Arabic commentators on Aristotle. Medieval Christian mysticism also shared the Neoplatonic influence of Sufis such as Ibn Arabi. Philosopher Frederick Copleston argued in 1950 that Dante's respectful treatment of Averroes, Avicenna, and Siger of Brabant indicates his acknowledgement of a "considerable debt" to Islamic philosophy.

Although this philosophical influence is generally acknowledged, many scholars have not been satisfied that Dante was influenced by the Kitab al Miraj. The 20th century Orientalist Francesco Gabrieli expressed skepticism regarding the claimed similarities, and the lack of evidence of a vehicle through which it could have been transmitted to Dante. Even so, while dismissing the probability of some influences posited in Palacios' work, Gabrieli conceded that it was "at least possible, if not probable, that Dante may have known the Liber scalae and have taken from it certain images and concepts of Muslim eschatology". Shortly before her death, the Italian philologist Maria Corti pointed out that, during his stay at the court of Alfonso X, Dante's mentor Brunetto Latini met Bonaventura de Siena, a Tuscan who had translated the Kitab al Miraj from Arabic into Latin. Corti speculates that Brunetto may have provided a copy of that work to Dante. René Guenon, in The Esoterism of Dante, rejected the influence of Ibn Arabi (direct or indirect) on Dante.

Literary influence in the English-speaking world and beyond

The work was not always so well regarded. After being recognized as a masterpiece in the centuries immediately following its publication, the work was largely ignored during the Enlightenment, with some notable exceptions such as Vittorio Alfieri; Antoine de Rivarol, who translated the Inferno into French; and Giambattista Vico, who in the Scienza nuova and in the Giudizio su Dante inaugurated what would later become the romantic reappraisal of Dante, juxtaposing him to Homer. The Comedy was "rediscovered" by William Blake – who illustrated several passages of the epic – and the romantic writers of the 19th century. Later authors such as T.S. Eliot, Ezra Pound, Samuel Beckett, C.S. Lewis and James Joyce have drawn on it for inspiration. The poet Henry Wadsworth Longfellow was its first American translator, and modern poets, including Seamus Heaney, Robert Pinsky, John Ciardi, and W.S. Merwin, have also produced translations of all or parts of the book. In Russia, beyond Pushkin's memorable translation of a few tercets, Osip Mandelstam's late poetry has been said to bear of the mark of a "tormented meditation" on the Comedy. In 1934 Mandelstam gave a modern reading of the poem in his labyrinthine "Conversation on Dante". Mikhail Lozinsky's translation of the poem, completed in 1945, is considered to be one of the greatest works of Russian poetry in the 20th century and arguably the best translation of any foreign - language poem into Russian ever.

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