Document Type : Research
Authors
1
Erfan Biabani, Ph.D. Student in Philosophy, University of Tehran, Tehran, Iran
2
Faculty Member of the Department of Western Philosophy, University of Tehran
10.30465/crtls.2026.54669.3024
Abstract
Descartes addresses the relationship between metaphysics and physics in the Preface to the Principles of Philosophy through the metaphor of the tree of knowledge. Raising this issue in the preface to a work intended to articulate this relationship systematically arises from Descartes’s philosophical system itself and accords with its structural coherence—a system in which metaphysics serves as the foundation of physics, without thereby reducing physics to the results of purely deductive reasoning. This article seeks to answer the question of how this relationship should be understood in light of the metaphor of the tree of knowledge. An examination of The World and the Principles of Philosophy shows that, unlike the first two levels in the formation of physics, which belong to the domain of pure metaphysics—that is, rational ideas and the propositions based upon them—and in which deductive inference proceeds on the basis of pure reason and the criterion of clarity and distinctness, the three laws of nature are not obtained solely through pure rational deduction from the preceding levels; hypotheses and experiments also play a decisive role in their formulation. Accordingly, the “rootedness” of physics in metaphysics does not mean that its laws are directly deduced from metaphysics, but rather that they rest upon a deeper foundation—a foundation that performs three roles in relation to physics: ontological, epistemological, and methodological.
Introduction
The relationship between metaphysics and physics in Descartes’s philosophy is commonly understood through the metaphor of the “tree of knowledge” presented in the Preface to the French edition of the Principles of Philosophy, where metaphysics constitutes the roots, physics the trunk, and the other sciences the branches. The metaphor itself, however, does not clarify precisely what it means for metaphysics to be the “root” of physics or what kind of relation obtains between metaphysical principles and physical laws. If the foundational status of metaphysics means that physical laws are logically and deductively derived from metaphysical principles, these laws should follow as necessary consequences of those principles. Yet Descartes’s actual procedure in formulating and justifying the laws of nature, particularly in The World and the Principles of Philosophy, does not support such an interpretation. In these works, hypotheses, observation, and experiment play a significant role alongside metaphysical principles. This study therefore seeks to clarify the precise sense in which metaphysics provides the foundation of Cartesian physics and to show how the tree-of-knowledge metaphor can be understood without reducing physics to a set of purely deductive consequences of metaphysics.
Materials & Methods
This study employs a conceptual and textual analysis of Descartes’s works, focusing primarily on the Meditations on First Philosophy, The World, and the Principles of Philosophy, together with relevant correspondence. It first examines Descartes’s dual use of the concept of “principle,” which may signify either a fundamental being or a self-evident general proposition. On this basis, three levels in the formation of Cartesian physics are distinguished. The first is the metaphysical foundation of physics, where purely rational ideas such as extension and motion are established. The second consists in the derivation of metaphysical propositions based on these ideas, including the connection of the first cause of motion and the conservation of its quantity with divine immutability. The third concerns the formulation of the laws of nature and their application to determinate bodies. The study then examines the justification of the three laws of nature in The World and the Principles of Philosophy, with particular attention to Descartes’s use of such notions as deduction, demonstration, explanation, and cognition, in order to assess whether these laws can properly be regarded as directly deduced from metaphysical premises.
Discussion & Result
The analysis demonstrates that the inferential relationship between metaphysics and physics is not uniform across these three levels. At the first level, extension is established as the principal attribute of corporeal substance and motion as its mode, on the basis of rational ideas and the criterion of clarity and distinctness. At the second level, principles such as the first cause of motion and the conservation of its quantity are formulated by reference to God and divine immutability within a rational and metaphysical structure. At these two levels, it is therefore legitimate to speak of a rational and deductive order. At the third level, however, where inquiry concerns determinate bodies and natural phenomena, pure rational deduction is no longer sufficient to determine the content of physical laws; hypothesis, observation, and experiment enter into the formation of natural science.
An examination of the laws of nature in The World makes this difference evident. The first law is associated with the general principle that nothing changes without a cause. The second is explicitly introduced as a hypothesis whose plausibility is supported by its agreement with observations. The third is justified through a combination of empirical evidence and an appeal to the constancy and immutability of divine action. In the Principles of Philosophy, likewise, although Descartes relates the laws of nature to divine immutability, his language does not necessarily imply their deductive derivation from this metaphysical attribute. Even where he speaks of “proof,” the text provides explanation and confirmation rather than a strictly deductive demonstration. Moreover, Descartes’s repeated appeals to everyday experience and particular observations indicate that physical laws at the third level cannot be regarded merely as formal consequences of metaphysical principles.
Metaphysics consequently performs three fundamental functions in relation to physics. First, its ontological function consists in determining the nature of matter as extension, accounting for the first cause of motion, and grounding the constancy of the quantity of motion in divine immutability. Second, its epistemological function consists in providing the theoretical foundation of certainty and securing the possibility of valid knowledge, although the form and degree of certainty attainable in natural science differ from metaphysical certainty. Third, metaphysics has a methodological function: it delimits the legitimate domain of scientific explanation by determining which entities, concepts, and modes of explanation may enter into physics. The rejection of obscure Scholastic forms and qualities and the restriction of natural science to quantitative and mechanical properties such as size, shape, motion, and the arrangement of parts are major consequences of this methodological function.
Conclusion
The tree-of-knowledge metaphor should not be understood as representing a strict logical deduction of physics from metaphysics. For metaphysics to constitute the “roots” of physics means that Cartesian natural science is grounded, justified, and delimited within a metaphysical framework; it does not mean that the specific content of every physical law follows necessarily from metaphysical principles. At the first two levels—the establishment of fundamental ideas and the derivation of metaphysical propositions based upon them—pure reason and deduction play a decisive role. At the third level, however, the laws of nature emerge through the interaction of these prior principles with hypothesis, observation, and experiment. What passes from metaphysics to physics, therefore, is not the specific content of physical laws but a foundation with three interconnected functions: ontological, epistemological, and methodological. Metaphysics thus provides the philosophical conditions for the formation of natural science and determines its conceptual boundaries and standards, while physics retains relative autonomy in its engagement with the natural world. Consequently, the proposition that “physics is deduced from metaphysics” is defensible only in an extended Cartesian sense: metaphysics provides the foundation, legitimacy, and conceptual framework of natural science, rather than rendering the laws of nature formal and necessary consequences of metaphysical principles.
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