تثليث المعدة
A guideline to divide the stomach's capacity into thirds — food, drink, and breathing room — rather than filling it entirely.
مَا مَلَأَ آدَمِيٌّ وِعَاءً شَرًّا مِنْ بَطْنٍ، بِحَسْبِ ابْنِ آدَمَ أُكُلَاتٌ يُقِمْنَ صُلْبَهُ، فَإِنْ كَانَ لَا مَحَالَةَ فَثُلُثٌ لِطَعَامِهِ، وَثُلُثٌ لِشَرَابِهِ، وَثُلُثٌ لِنَفَسِهِ
“No vessel is filled worse than the stomach. A few mouthfuls are enough to keep a person upright; if more is unavoidable, a third for food, a third for drink, and a third for breath.” (meaning, not a literal translation)
The hadith's structure is notable: it opens with an ideal (a few mouthfuls) before offering the thirds-division as a fallback for when more is genuinely needed — read together, it's less a rigid rule and more a hierarchy of preference, from minimal to merely moderate.
Ibn al-Qayyim treats this as the single most important dietary principle in the corpus — overfilling the stomach was, in his Galenic framework, the root cause of most digestive and humoral imbalance. He connects it directly to several other entries on this site: the caution against overeating recurs in his discussion of dates, honey, and other foods, always circling back to this same portion principle as the underlying factor.
Modern nutritional science doesn't use a literal thirds-of-the-stomach framework, but portion moderation and avoiding habitual overeating remain consistent recommendations across contemporary dietary guidance for digestive comfort and long-term metabolic health.