The Sixteen Accidents (al-aʿrāḍ)
Definition
The sixteen accidents (Arabic al-aʿrāḍ, 'the things that befall') are Sahl ibn Bishr's catalogue, in §5 of his Introduction, of the ways planets relate as they advance or hinder a matter. They are: the approach (al-iqbāl), the falling-back (al-idbār), the connection (al-ittiṣāl), the departure (al-inṣirāf), the transmission (an-naql), the collection (al-jamʿ), the prohibition (al-manʿ), the reception (al-qubūl), not-reception, void of movement, the return (ar-radd), the pushing of strength (dafʿ al-quwwah), the pushing of arrangement and nature, strength, weakness, and the condition of the Moon.
In Tradition
Arabic-Persian astrologers treat the sixteen accidents as the working vocabulary of horary and electional judgment: every way a matter can perfect or fail is named among them. Sahl's list consolidates the Hellenistic applying-and-separating-aspect doctrine into a single numbered framework, and the later horary techniques — transmission (translation) of light, collection, reception, prohibition, return, and the pushing operations — are individual accidents drawn from it. The condition of the significators is read through which accidents hold between them.
In Practice
In a horary or electional chart, identify the significators — usually the lord of the Ascendant, the lord of the house for the matter asked about, and the Moon — then read the accidents holding between them in the order they occur. Watch first whether each significator is approaching or falling back; then whether they connect by an applying aspect or are departing; then whether reception is present, the receiving planet welcoming the applying one by sign or dignity. A connection with reception that completes uninterrupted perfects the matter, while prohibition (a third planet completing first), a return, or a void of movement interrupts it. The transmission of light carries a connection between two significators through a faster go-between; the collection joins both to a slower third planet; the pushing operations show how dignity and quality pass between planets. Judge the outcome from which accidents carry the matter through and which cut it off, weighing each planet's strength or weakness and the special condition of the Moon.
Historical Origin
The sixteen accidents are enumerated in Sahl ibn Bishr's Introduction (§5.0-§5.16, early 9th century), preserved in Benjamin N. Dykes's Works of Sahl & Masha'allah (trans. from the Arabic, with parallel Arabic-Latin text). Sahl's list is the foundational Arabic systematization of the applying/separating-aspect doctrine that runs through all later medieval horary, including Bonatti and Lilly.