Review of The Rational Ontological Argument: Modality, Ontology and God

Sijuwade, Joshua R. The Rational Ontological Argument: Modality, Ontology and God. London: Bloomsbury Academic, 2025, pp. 288, £85, Hardback.

Sijuwade displays an excellent grasp of the literature on the ontological argument and on modal metaphysics and epistemology, and uses it to develop an innovative and original ontological argument for God’s existence. This book considerably advances discussion of a longstanding argument for God’s existence, and it is a terrific contribution to the field of philosophy of religion.

Joshua Sijuwade holds the position of Teaching Fellow in Philosophy at the University of Birmingham, and specialises in philosophy of religion and analytic theology. His research record is nothing short of remarkable, and he has published numerous articles in some of the top journals in his fields of specialisation, such as Religious Studies, the International Journal for Philosophy of Religion, and Faith and Philosophy. Sijuwade’s first book, Analytic Theism, was published in the Routledge Studies in the Philosophy of Religion series in 2024, and the book that we are focusing on in this review, The Rational Ontological Argument, was published in the Bloomsbury Studies in Philosophy of Religion series in 2025.

The Rational Ontological Argument develops a new version of the ontological argument based on a metaphysical framework called Rational Modal Realism (RMR) that is also developed in this work. The introduction outlines the different versions of the ontological argument produced throughout the history of philosophy and canvasses the critiques raised against them. In chapter 1, modal realism – the view that possible worlds exist – is discussed, and competing views that either suggest these possible worlds exist abstractly or suggest they exist concretely like the actual world are considered and critiqued. Modal rationalism – the view that knowledge about modal properties can be acquired primarily through a priori reasoning – is outlined and defended in chapter 2. Chapter 3 introduces RMR, a theory that draws on both modal realism and modal rationalism; chapter 4 provides justification for RMR; and then chapter 5 is where Sijuwade constructs and argues for his new ontological argument that uses RMR as its foundation.

RMR has two key entailments that distinguish it from the competing versions of modal realism that Sijuwade outlines. Firstly, it posits the existence of an inner and outer sphere of possible worlds, with the inner being composed of concrete worlds identified with those of the Everettian or Many-Worlds Interpretation of Quantum Mechanics, and the outer is constituted of abstract fictional worlds. Secondly, this framework proposes an understanding of worlds in which they are concrete regions of space-time that objects exist in by being located at specific regions of space-time, and objects can have parts and properties in different worlds. The possible worlds in RMR are all interconnected and potentially overlapping. The modal rationalism used to acquire knowledge of the modal properties involved in RMR involves a two-tiered approach: the first stage involves using primary conceivability to discern what’s primarily possible, and the second stage involves using secondary conceivability and counterfactual reasoning to discern what’s secondarily possible. Thus, this modal framework has both metaphysical and epistemological elements.

The Rational Ontological Argument develops a new version of the ontological argument based on a metaphysical framework called Rational Modal Realism (RMR) that is also developed in this work. The introduction outlines the different versions of the ontological argument produced throughout the history of philosophy and canvasses the critiques raised against them.

Sijuwade’s ontological argument is built on this modal framework, and he calls it the Rational Modal Realist Ontological Argument. Sijuwade constructs his argument using divine immensity instead of divine necessity, making his version very different from competing forms of the argument. Due to God’s divine immensity, God’s presence is taken to have no spatial limits. Thus, if God, a maximally great being with divine immensity, exists in one of the concrete or abstract worlds within the pluriverse, due to the fact that God’s existence would have no spatial limits God would exist in every world within the pluriverse, including the actual world. Thus, God exists in the actual world.

This book is a must read for any scholar interested in traditional arguments for God’s existence. Sijuwade masterfully builds on the existing literature on the ontological argument before constructing his own innovative version of the argument based on a modal framework that he also develops in this work. Thus, there is much in this work that is original and interesting. It will primarily appeal to philosophical and analytic theologians. However, the extensive discussion of modal realism and modal rationalism and the construction of RMR will be of interest to analytic philosophers interested in modal metaphysics and epistemology. The use of the Many-Worlds Interpretation of Quantum Mechanics should also appeal to science-engaged theologians, as it provides an excellent example of how theories from modern science can be used in philosophical discussions of God.

This book includes discussion of complex philosophical literature, particularly in its discussion of modal realism, modal rationalism, and theories of location. Thus, it certainly presents a challenge for students and scholars whose primary area of expertise is philosophical theology instead of analytic metaphysics and epistemology. However, Sijuwade does an excellent job of canvassing and assessing the different views in these areas, so the book is accessible even for those who lack background knowledge in these areas.

There are several ways that scholars and students might choose to engage with this work. The first is by assessing the philosophical framework of RMR independently of its use in the ontological argument. This will require assessing its philosophical merits as a theory of modal metaphysics and modal epistemology. The second is by assessing Sijuwade’s ontological argument itself, and considering the potential advantages and disadvantages of this version when contrasted with rival versions. The third is by going beyond this and considering the implications of the ideas Sijuwade uses in the construction of his argument for other debates about God. For example, the book involves considerable discussion of divine location, so it would be fruitful to bring it into dialogue with the contemporary debate on divine presence.

However one chooses to engage with the book, it is certainly a book that is worth engaging with. Sijuwade displays an excellent grasp of the literature on the ontological argument and on modal metaphysics and epistemology, and uses it to develop an innovative and original ontological argument for God’s existence. This book considerably advances discussion of a longstanding argument for God’s existence, and it is a terrific contribution to the field of philosophy of religion.