The word's field
Ṣadr names first the chest, the physical breast of the body, and from that anatomical base it opens outward into one of the Quran's principal words for the human interior. Where qalb (see qalb) is the heart proper — the organ of understanding and the site of belief or its sealing — ṣadr is the wider cavity that holds the heart, and the language of the tradition keeps the two in a nesting relation: the breast is the chamber, the heart is what sits within it. The plural, ṣudūr, is where the word does most of its Quranic work, standing for the collective inner life of people in general, the hidden contents that only God fully knows.
The word's semantic career runs along two lines the commentarial tradition reads together. On one line ṣadr is a space that can be opened or narrowed: an expanded breast is made receptive, brought to ease and capacity, while a constricted breast is the mark of resistance, distress, or refusal. On the other line ṣadr is a container of what is concealed — intentions, secrets, the true state of a person the face may not show. Classical Arabic lexicography also preserves the plainly spatial sense, the ṣadr of a thing being its foremost or leading part, so the word carries a quiet directional charge: the breast is the front of the person, what faces outward even as it hides what is within. In its Quranic deployment the term sits close to qalb and to nafs (see nafs), but it is distinctly the architectural word among them — less the faculty than the room the faculty occupies.
In the corpus
The Logoi Arabic corpus is the complete Quran, indexed as a single work. Within it ṣadr is attested 44 times, all 44 occurrences falling within that one work — the whole of the received text, not a narrow sample. Because the corpus is one book, there is no cross-work distribution to weigh: the figure to read is the internal one. Forty-four occurrences place ṣadr among the Quran's recurrent vocabulary for the interior person, and the surface forms recovered by concordance show why the plural carries the theological weight. Singular ṣadr appears where an individual breast is opened, straitened, or set at rest; the plural ṣudūr dominates the passages about what God knows of the collective human inside. The sampled loci reach across the text: the word surfaces in early Meccan material and in the long Medinan suras alike, in eschatological passages about disclosure and in the closing sura's account of whispered insinuation. It is not confined to one register but recurs wherever the Quran turns to the hidden interior and its exposure.
Canonical moments
At The Quran 16:106 the singular ṣadr carries the verse's whole distinction. The one compelled to utter unbelief under duress is excused so long as the breast remains at rest, secure in faith; condemnation falls on the one whose breast is opened wide to disbelief. Here ṣadr is the true register of a person against the coerced word of the tongue — the interior that God reads beneath compulsion.
At The Quran 114:5, the final verse of the final sura, the whisperer insinuates in the breasts of mankind. The plural ṣudūr names the site where suggestion lands and works — not the ear, not the reasoning heart directly, but the whole interior cavity, the vulnerable inside into which doubt is breathed. The Quran ends on this word for the human interior as the contested ground of the spiritual life.
At The Quran 100:10 the word turns eschatological: what is in the breasts is brought out, made to appear, at the hour when the graves are overturned. Here ṣudūr is the container of the concealed, and the resurrection is figured as the moment its contents are disclosed. The breast that hides in this life is emptied of its secret in the next.
At The Quran 11:5 the motion is the human attempt at concealment: people fold up their breasts to hide from God, and the verse answers that He knows what they conceal and what they reveal even as they draw their garments over themselves. The word appears twice in this single verse, once as ṣudūr and once in a related plural surface, underlining the futility it describes — the breast is exactly the place that cannot be closed against the one who made it.
The word's world
Ṣadr belongs to a family of interior words that every scriptural tradition assembles, and its particular note is spatial. Within its own wing it is the chamber to qalb's occupant (see qalb), and it stands near nafs (see nafs), the self that must be examined, and near ruh (see ruh), the breathed spirit — but ṣadr alone is the architecture rather than the faculty or the breath. That architectural sense reaches across traditions most directly to the Hebrew organ-words: to leb (see leb), the heart that houses mind, will, and courage together, and to basar (see basar), the flesh that is the outward, mortal side of the person. The Quran's recurring image of what lies in the breasts — known to God, hidden from other people, disclosed at the resurrection — answers the biblical conviction that the heart is what only God can search.
The Greek and Latin siblings sharpen the contrast. Where cor (see cor) names the heart as the seat of thought and feeling, and where thumos (see thumos) locates a surging inner life in the chest of the Homeric warrior, ṣadr is more nearly the room than the resident: the physical breast made a figure for the concealing, receptive, exposable interior. Its two Quranic movements — the breast that opens to receive, the breast that hides what will one day be brought to light — give the Arabic tradition a precise vocabulary for the inside of a person considered as a space that God enters, reads, and finally lays open.
Grounding: corpus figures and citations from the live Logoi corpus record (receipt soul-word-journey-v0). Cross-references: qalb, nafs, ruh, leb, basar, cor, thumos.