Canto XXIII. The three poets continue their progress around the cornice, and meet the souls of the gluttonous, who are emaciated. One, Forese Donati, is a friend of Dante, and explains that he has made rapid progress up the mountain because of the prayers of his widow. He prophesies Dante’s problems in Florence, and punishments for that city.

In the Summa Theologiae (1a.2ae.148.1) St Thomas asks whether gluttony really is a sin. Afterall, the drive towards food and drink is a basic human instinct. He concludes that it is, because the root and beginning of the sin is not in the drive itself, but begins when this becomes detached from reason. ‘Gluttony denotes, not any desire of eating and drinking, but an inordinate desire.’ By inordinate desire he means that which ‘ leav[es] the order of reason, wherein the good of moral virtue consists: and a thing is said to be a sin through being contrary to virtue.’ From what we have seen before, we might depart from virtuous use of food and drink in various ways. Mostly we tend to eat too much, but I once heard a Dominican friar give the example of someone who, at a surprise party, fails to join in whole-heartedly. This too is a failure in temperance, and is a sin which might be purged amid the gluttonous.

Forese shows Dante the tree of the gluttonous.

http://www.bodley.ox.ac.uk/dept/scwmss/wmss/medieval/jpegs/holkham/misc/48/1000/04800481.jpg