| Summary: | <p>This is a study of the metaphysics of necessary existence in the thought of the thirteenth century Persian polymath Naṣīr al-Dīn Ṭūsī (d. 672/1274). It argues that Ṭūsī is part of an important conceptualist turn within the interpretation of Avicenna's metaphysics around the thirteenth century, and that two key principles within his metaphysical scheme provide the essential means by which he defends the tenor of his predecessor's general ontology against its detractors in the aporetic tradition. These principles, each one of which is presented in their historical manifestation, are the (1) the ambiguity of existence (tashkīk al-wujūd) and (2) the being of reason (<em>al-i'tibār al-'aqlī</em>). The integrity of these concepts as well as the importance they have to the verification (<em>taḥqīq</em>) of Avicenna's metaphysics is demonstrated with reference to the single and undoubtedtly most important idea within the Avicennan metaphysic, namely, its conception of a being which is necessary of existence in itself (<em>wājib al-wujūd bi-dhātihi</em>). Avicenna makes two fundamental claims about the nature of such a being: that it is "sheer oneness" and that it is a being the essence of which is identical to its existence. The former establishes the unicity of the divine essence, while the latter affirms the identity of its essence and existence (māhiyyatuhu inniyyatuhu), both of which are the consequence of it being necessary in itself. Avicenna's detractors in the theological tradition rejected both of these claims by confuting the notion of divine aseity. I consider the cumulative objections raised by three of Avicenna's major critics, namely Ghazālī (d. 1111), Shahrastānī (d. 1153) and Rāzī (d. 1210)—and then demonstrate how each of these principles in Ṭūsī's thought effectively defends the Avicennan conception of the Necessary Existent.</p> <p>By a gradual discovery of the fecundity of rationate beings since (at least) the time of ʿUmar Khayyām, I argue further that the concept of a being of reason (<em>al-i'tibār al-'aqlī</em>) is the single most important development in the mainstream post-Avicennan tradition. The second part of this thesis is therefore devoted to the notion of mental existence (<em>al-wujūd al-dhihnī</em>), a realm of being which ontologically grounds the subsistence of rationate being. The final part of this dissertation provides a discussion of Ṭūsī's innovative solution to the problem of correspondence; that is, how does a being of reason correspond to reality if per definiens it is a being incapable of mind-independent existence? This requires a foray into the metaphysics of truth and one of the most important philosophical ideas in the whole of post-classical Islam: 'the thing itself' (<em>nafs al-amr</em>). I examine the novel contributions which Ṭūsī has made for the inauguration and subsequent development of this concept in the post-Avicennan tradition. The trajectory of this dissertation may be thus be described as journey from 'being Necessary' to 'being mental' to 'being true'.</p>
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