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)
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.