Franklin links excessive eating with reduced mental sharpness. A “full belly” represents more than simply having eaten enough; it suggests overindulgence and the physical sluggishness that can follow it. The “dull brain” is a mind that is less alert, energetic or capable of sustained thought.

The saying reflects Franklin’s broader emphasis on moderation and self-discipline. Physical pleasure is not rejected, but excess is presented as having practical consequences. Comfort and appetite can become distractions when they interfere with attention, work or judgement. The quotation can therefore be read as advice to regulate bodily habits in order to preserve mental effectiveness. Its point is not a medical claim that eating necessarily impairs intelligence, but a concise observation about the familiar experience of feeling lethargic after excess and about the wider value of restraint.