The present study reconstructs psychological, linguistic and ontological aspects of Mullā Ṣadrā's philosophy in the light of Brentano's theory of intentionality. Brentano used intentionality as a psychological term to denote the 'mental' as opposed to the 'natural'. Later, with Meinong, intentionality took an ontological commitment to assenting that 'there are things that do not exist'. The chapters that discuss Ṣadrā's philosophy reflect the two aspects with an investigation for the production process of intentional objects and an investigation of the status of these objects in ontology. The main aim of the research is to give an internalist and monist account for the nature of intentionality demonstrating an alternative approach to the concepts of existence and the soul. Ontologically, there is only one reality (existence) and nothing is left outside it. Accordingly, intentional objects are mental beings that are at a lower level of existence (wujūd ẓillī). The principles behind the monist ontology are: first, the gradational ontology (tashkīk) that all things are determined beings (mutamayyiz) and they are manifestations of a single reality at different levels of intensity (mutashakkik), and, second, the simplicity principle (basīṭ al-ḥaqīqa) in which existence is a simple reality that comprehends all beings whilst being the principle of multiplicity at the same time. Accordingly intentional objects are a level of existence, and share same reality. Epistemologically, all knowledge processes including external senses are regarded as internal processes in which the causal effect of the extra-mental object is reduced to being an accidental preparatory tool and faculties for the soul. Perception is always completed with the touch of imagination and the real object of perception is internally created. The soul is not the receiver of forms, but is the active agent. Moreover, the soul undergoes substantial change as the objects are being produced. The soul is then not a container of forms. It is rather the case that the forms themselves construct the soul. The last point is that knowledge is a mode of existence. This mode of being (knowledge) indeed is the very existence of the human soul. In this explanation, the soul is neither material nor immaterial per se: the soul starts her journey as a material substance and becomes more delicate and immaterial through her journey. The soul's journey is made possible with the preparatory role of the processes of perception. Intentionality is soul's action of creating mental forms. The products are identical to soul since soul and knowledge are identical. Consequently, intentional objects are dependent on the soul in their presence and creation.