Neuroscience
Hearts to Reason
Scientific Context
The Intrinsic Cardiac Nervous System
The understanding of the heart as nothing more than a mechanical pump that unthinkingly circulates blood was challenged in the early 1990s, when the neurocardiologist J. Andrew Armour, then at the University of Montreal, proposed that the heart contains its own intrinsic nervous system, a network of neurons embedded in the heart wall that can process information and regulate cardiac function independently of the brain. The system, which Armour called the intracardiac nervous system, was described in a 1991 paper. The heart, in Armour's framing, is a little brain capable of perception.
The neurons in the heart wall number in the tens of thousands, with the most-cited count from Armour's later work landing at approximately 40,000. The system operates semi-independently: a transplanted human heart, severed from the brain during surgery, will beat in its new chest for the life of the recipient without ever being reconnected — its own decision-making layer.
The heart also talks to the brain more than the brain talks to it. The signals travel upward through the vagus nerve, carrying information about pressure, rhythm, and chemical state into brain regions involved in emotional processing, decision-making, and memory. The proportion has been measured at roughly four heart-to-brain signals for every brain-to-heart signal. Stephen Porges extended the picture with polyvagal theory, showing that the variability of the heart's beat between breaths is a measurable index of emotional state. The heart is also a hormone-producing organ: in the early 1980s, Adolfo Bold and his colleagues at the University of Ottawa discovered atrial natriuretic peptide, a hormone released by the atria that crosses into the brain and acts on regions of fluid balance and stress.
What the picture adds up to is a heart that is not a mere mechanical pump. The discovery has shifted the working understanding of the heart-brain relationship from a top-down command structure, in which the brain instructs the heart, to a layered system in which the heart is a participant.
Islamic Context
أَفَلَمْ يَسِيرُوا فِي الْأَرْضِ فَتَكُونَ لَهُمْ قُلُوبٌ يَعْقِلُونَ بِهَا أَوْ آذَانٌ يَسْمَعُونَ بِهَا فَإِنَّهَا لَا تَعْمَى الْأَبْصَارُ وَلَٰكِن تَعْمَى الْقُلُوبُ الَّتِي فِي الصُّدُورِ
So have they not travelled through the earth and have hearts by which to reason and ears by which to hear? For indeed, it is not eyes that are blinded, but blinded are the hearts which are within the breasts.
— Quran 22:46
The verse opens with a rhetorical question: afalam yasīrū fī al-arḍi fa-takūna lahum qulūbun yaʿqilūna bihā — have they not travelled through the earth and have hearts by which to reason? The verb yaʿqilu names a cognitive act, the kind of understanding that grasps meaning rather than mere sensation. The grammar is direct: it is the qulūb that does the reasoning. The verse does not say hearts are associated with reasoning. Al-Qurtubi (d. 1273) notes in his commentary that this attribution is not metaphorical: the Qur'an is naming what the heart does, not pointing elsewhere through the heart's name.
Al-Razi (d. 1210) observes that the verb yaʿqiluna in this verse encompasses all forms of understanding — perception of signs, recognition of patterns, the kind of comprehension that the word also carries in the cognate form ʿaql. The travellers through the earth are charged not with seeing or hearing in particular, but with using the full cognitive capacity that the heart, in this register, names.
The second half of the verse draws a sharp distinction. The eyes, the verse says, are not what is blind. The blind thing is the heart. It exonerates the eyes — the deficiency is not in seeing but in processing what is seen — and locates the seat of that processing precisely, in the organ whose name has been doing the cognitive work throughout the verse. The Qur'an is naming a place in the body, and the place it names is the heart.
Ibn Kathir (d. 1373) reads the specification fī al-ṣudūr — "in the breasts" — as a deliberate anatomical emphasis rather than a tautology. The verse, in his reading, is not speaking of the inner life in the abstract but is locating it in a specific organ in a specific place in the body. Some of the early commentators read the specification as obvious, naming the heart's location for emphasis but adding nothing. The more interesting reading takes the specification as deliberate: the verse's precision about location is part of its claim, not a redundant aside.
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The Connection
The Qur’an identifies the heart as the primary seat of reasoning and cognitive perception, situating this capacity within the physical organ located in the chest. This aligns with modern neurocardiology, which has revealed that the heart is not a simple pump, but a complex organ with an intrinsic nervous system capable of independent information processing. The discovery of this "little brain" in the heart—which consistently sends data to the brain—shifts the understanding of cardiac function from mechanical output to active cognition.
