In philosophy, a theory of everything (ToE) is an ultimate, all-encompassing explanation or description of nature or reality.Rescher, Nicholas (2006a). "Holistic Explanation and the Idea of a Grand Unified Theory". Collected Papers IX: Studies in Metaphilosophy.Rescher, Nicholas (2006b). "The Price of an Ultimate Theory". Collected Papers IX: Studies in Metaphilosophy. (Googlebooks preview)Walker, Mark Alan (March 2002). "Prolegomena to Any Future Philosophy". Journal of Evolution and Technology Vol. 10. It aspires to answer the most fundamental questions: why there is anything at all, why reality is intelligible, and why the laws of nature take the form they do. The term, borrowed from physics, where the search for a final unifying framework is ongoing, has been adopted by philosophers to discuss both the viability of such a project and the properties a completed explanation would need to possess. A philosophical ToE is not a single equation but a comprehensive conceptual structure that integrates metaphysics, epistemology, ethics, and the philosophy of science into a coherent whole.

Historical precursors

The ambition to construct an all-embracing system of thought is as old as philosophy itself. In ancient Greece, Plato's theory of Forms and Aristotle's metaphysical and scientific treatises attempted to account for the totality of being. Plato's vision of a transcendent realm of perfect archetypes underlying the visible world and Aristotle's systematic categorization of substances, causes, and purposes set the pattern for later system-builders.

During the Hellenistic period, the Stoics developed a complete system encompassing logic, physics, and ethics, in which the universe was a living rational whole governed by the logos. Plotinus' Neoplatonism offered a grand metaphysical hierarchy emanating from the One. These ancient systems share the conviction that human reason can grasp the ultimate principles of reality.

In the medieval period, thinkers like Thomas Aquinas integrated Aristotelianism with Christian theology, producing what they regarded as a complete account of God, creation, and human nature. The Scholastic tradition aimed at a summa of all knowledge, a total system harmonizing faith and reason.

The early modern period (17th and 18th centuries) saw the rise of rationalist system-building, often equating systematic completeness with the power of a priori reason. Descartes' dualism sought to derive the fundamental principles of mind and matter from clear and distinct ideas. Spinoza's monism, presented in geometric order in his Ethics, argued that a single substance, God or Nature, unfolds according to necessary laws. Leibniz's monadology posited an infinity of simple, non-interacting substances pre-harmonized by God, offering a complete account of reality from the simplest elements upward.

In the 19th century, Georg Wilhelm Friedrich Hegel's absolute idealism attempted to demonstrate that all of reality is the historical and logical development of Spirit coming to know itself. Hegel's system was perhaps the most ambitious ToE yet, purporting to derive the necessary structure of reality from pure logic. Arthur Schopenhauer offered a competing world-system based on the metaphysical Will. Later, Herbert Spencer attempted to synthesize biology, psychology, sociology, and ethics under the principle of evolution, creating what he called a "Synthetic Philosophy".

Alfred North Whitehead's process philosophy in the early 20th century provided another large-scale metaphysical system, one in which becoming and relation are fundamental rather than static substance. Whitehead's Process and Reality aims to be a comprehensive cosmology, explaining everything from quantum physics to the nature of God. Each of these thinkers attempted, in their own way, to answer all the important questions in a single, interconnected framework.

The later 20th century saw a revival of systematic metaphysics in the analytical tradition through the work of philosophers like Peter Strawson (descriptive metaphysics) and David Lewis (modal realism). Strawson's Individuals (1959) attempts to map the most general features of our conceptual scheme, while Lewis's On the Plurality of Worlds (1986) offers a comprehensive modal realism that explains possibility, necessity, causation, and mind in terms of concrete possible worlds. While these works did not always present themselves as full-fledged ToEs, they shared the ambition of providing comprehensive frameworks for understanding reality.

Comprehensive philosophical systems

The "system building" style of metaphysics attempts to answer all the important questions in a coherent way, providing a complete picture of the world. The philosophies of Plato and Aristotle could be said to be early examples of comprehensive systems. In the early modern period (17th and 18th centuries), the system-building scope of philosophy is often linked to the rationalist method of philosophy, that is the technique of deducing the nature of the world by pure a priori reason. Examples from the early modern period include Leibniz's monadology, Descartes's dualism, and Spinoza's monism. Hegel's absolute idealism and Whitehead's process philosophy were later systems.

More recent examples of system-oriented metaphysical approaches include structural-systematic philosophy, associated with the work of Lorenz B. Puntel and Alan White. Puntel develops this approach in Structure and Being: A Theoretical Framework for a Systematic Philosophy (2008) and Being and God (2011), which present a systematic metaphysical framework centered on structural relations. White draws on this project and extends it in Toward a Philosophical Theory of Everything (2014). Like earlier system-building philosophies, these works aim to articulate a comprehensive and internally coherent account of being.

Other philosophers do not believe philosophy should aim so high. Some scientists think a more mathematical approach than philosophy is needed for a ToE. For instance, Stephen Hawking wrote in A Brief History of Time that even if we had a ToE, it would necessarily be a set of equations. He wrote, "What is it that breathes fire into the equations and makes a universe for them to describe?"As quoted in Mariano Artigas, The Mind of the Universe, Templeton Foundation Press, 2001, p. 123. Similarly, physicist Steven Weinberg's Dreams of a Final Theory (1992) explores the longing for a final, beautiful set of physical laws while cautioning that such a theory would not answer all philosophical questions.

Integral theory and contemporary grand narratives Another contemporary attempt at a comprehensive synthesis is integral theory, developed by Ken Wilber. Wilber's work attempts to integrate insights from science, psychology, spirituality, and cultural studies into a single framework he calls the "AQAL" (All Quadrants, All Levels) model. It posits that reality consists of holons (whole/parts) arranged in nested hierarchies of development across multiple dimensions. While integral theory has a significant popular following, it has been criticized by academic philosophers for its lack of rigorous argumentation and its tendency to conflate descriptive and prescriptive claims. Nevertheless, it represents a modern incarnation of the system-building impulse, demonstrating that the dream of a unified picture of everything remains alive.

Nicholas Rescher's analysis

Properties and the impasse of self-substantiation In "The Price of an Ultimate Theory", originally published in 2000, Nicholas Rescher specifies what he sees as the principal properties of a Theory of Everything and describes an apparent impasse on the road to such a theory.

Principle of sufficient reason First, he takes as a presupposition the principle of sufficient reason, which in his formulation states that every fact x has an explanation y:

where E predicates explanation, so that x E y denotes "y explains x". This expresses that the explanatory relation E is left-total.

Comprehensiveness Next, he asserts that the most direct and natural construction of a Theory of Everything T would confer upon it two crucial features: comprehensiveness and finality. Comprehensiveness says that wherever there is a fact x, T affords its explanation:

In particular, T explains itself (although it may be explained by one or more other facts as well):

Finality Finality says that as an "ultimate theory", T has no deeper explanation:

so that the only conceivable explanation of T is T itself.

Noncircularity Rescher notes that it is obviously problematic to deploy a theory for its own explanation; at the heart of the traditional conception of explanatory adequacy, he says, is a principle of noncircularity stating that no fact can explain itself:

Impasse The impasse is then that the two critical aspects of a Theory of Everything, comprehensiveness and finality, conflict with the fundamental principle of noncircularity. A comprehensive theory which explains everything must explain itself, and a final theory which has no deeper explanation must, by the principle of sufficient reason, have some explanation; consequently it too must be self-explanatory. Rescher concludes that any Theorist of Everything committed to comprehensiveness and finality is bound to regard noncircularity as "something that has to be jettisoned". But how, he asks, can a theory adequately substantiate itself?

Ways forward Rescher's proposal in "The Price of an Ultimate Theory" is to dualize the concept of explanation so that a fact can be explained either derivationally, by the premises which lead to it, or systemically, by the consequences which follow from it. With derivational explanation, a fact t is explained when it is subsumed by some prior, more fundamental fact t. With systemic explanation, t is explained when it is a "best fit" for its consequences, where fitness is measured by uniformity, simplicity, connectedness, and other criteria conducive to systemic integration. Rescher concludes that while a theory of everything cannot be explained derivationally (since no deeper explanation can subsume it), it can be explained systemically by its capacity to integrate its consequences.

In his 1996 book The Conscious Mind, David Chalmers argues that a theory of everything must explain consciousness, that consciousness does not logically supervene on the physical, and that therefore a fundamental theory in physics would not be a theory of everything. A truly final theory, he argues, needs not just physical properties and laws, but phenomenal or protophenomenal properties and psychophysical laws explaining the relationship between physical processes and conscious experience. He concludes that "[o]nce we have a fundamental theory of consciousness to accompany a fundamental theory in physics, we may truly have a theory of everything." Developing such a theory will not be straightforward, he says, but "it ought to be possible in principle."

In "Prolegomena to Any Future Philosophy", a 2002 essay in the Journal of Evolution and Technology, Mark Alan Walker discusses modern responses to the question of how to reconcile "the apparent finitude of humans" with what he calls "the traditional telos of philosophy—the attempt to unite thought and Being, to arrive at absolute knowledge, at a final theory of everything." He contrasts two ways of closing this "gap between the ambitions of philosophy, and the abilities of human philosophers": a "deflationary" approach in which philosophy is "scaled down into something more human" and the attempt to achieve a theory of everything is abandoned, and an "inflationary", transhumanist approach in which philosophers are "scaled up" by advanced technology into "super-intelligent beings" better able to pursue such a theory.

Criticism