The Argument of the Truthful (burhān al-ṣiddīqīn) is Ibn Sīnā's distinctive demonstration of the existence of God, presented most concisely in the Ishārāt wa-l-Tanbīhāt. The argument is called "of the truthful" because, as Ibn Sīnā explains, it proceeds neither from the createdness of the world nor from the order of nature, but directly from the consideration of existence itself, requiring nothing but reflection on what existence is. From the bare fact that something exists, the argument derives the existence of a necessary being whose existence is identical with its essence.
The structure of the argument is highly compressed. If any existence at all is granted, then we may consider the whole of what exists. This whole either exists necessarily or contingently. If contingently, it requires an external explanation; but there is nothing external to the whole of what exists, so the external explanation must itself be a being whose existence is necessary. This necessary being cannot be a part of the contingent whole, since it would then need to explain itself. Therefore there exists a necessary being whose existence is identical with its essence — what Ibn Sīnā calls wājib al-wujūd bi-dhātihi, "the necessarily existent in itself." The argument's elegance lies in its operating purely at the level of existence and modality, without empirical premises about causation, motion, or the temporal beginning of the world.
The argument has been the subject of intense exegetical and philosophical debate. Mullā Ṣadrā considered it the deepest demonstration in the philosophical tradition and reformulated it in terms of the principal reality of existence (aṣālat al-wujūd). Critics within the Islamic tradition, including al-Ghazālī in certain readings and later Ashʿarī theologians, have questioned the move from the modal categories of necessity and contingency to the traditional attributes of God. Contemporary scholarship by Toby Mayer, Peter Adamson, Damien Janos, and others has examined the argument's structure and its relation to Aristotelian metaphysics, while debating whether it should be considered a cosmological argument at all or rather a distinct a priori argument from the nature of existence itself.
Among other formulations in the cosmological family, the burhān al-ṣiddīqīn differs from the First Cause and Thomistic versions in not depending on the causal structure of the world or the impossibility of infinite regress. It differs from the Kalam version, which relies on temporal beginning. It is most closely related to Ibn Sīnā's own burhān al-imkān wa-l-wujūb, with which it shares the modal categories of contingency and necessity, though it differs in its even more direct derivation from existence itself. It anticipates the Leibnizian and contemporary Contingency arguments by several centuries, providing a Western reformulation of insights first systematized in the Islamic peripatetic tradition.