Philosophical treatise: De Motu

Overview
George Berkeley's Latin "De Motu" (1721) is a tightly focused philosophical interrogation of the concepts that underpin the Newtonian account of motion. Rather than offering new mathematical mechanics, Berkeley examines the assumptions behind notions such as absolute space, absolute motion, and the causal status assigned to bodies in natural philosophy. The treatise reads as a philosophical companion to contemporary physics: attentive to empirical practice, skeptical about metaphysical extrapolations, and shaped by Berkeley's wider immaterialist commitments.
Berkeley treats motion not as a brute physical substance but as a concept loaded with metaphysical presuppositions. He draws attention to how mundane descriptions of moving bodies presuppose relations among observed bodies and minds, and he presses the methodological limits of invoking unobservable entities to account for observed regularities.

Critique of Absolute Space
A central target is Isaac Newton's doctrine of absolute space. Berkeley rejects the idea that space can be a real, independently existing container whose properties secure absolute rest and absolute motion. He argues that appeals to absolute space are unnecessary to make sense of observable phenomena and that they rest on questionable metaphysical assumptions. Berkeley challenges attempts to infer unperceived substrates from the behavior of bodies, insisting that the supposed explanatory gain does not justify positing a hidden, metaphysically thick arena.
Berkeley scrutinizes Newtonian thought experiments that aim to show the reality of absolute motion. He maintains that the phenomena invoked to establish absolute motion, such as centrifugal effects, admit relational explanations and that invoking absolute space merely disguises the reliance on relations among perceivable bodies and their manifested effects.

Relational Account of Motion
Berkeley advances a relational approach: motion is to be understood in terms of change of position relative to other bodies and to the system of relations that constitute a sensible world. He emphasizes that descriptions of motion are descriptions of orderings and courses of sensible phenomena rather than pointers to an underlying substantial motion through an invisible medium. This relational perspective aligns Berkeley with earlier criticisms by Leibniz and provides an alternative to the substantival reading of Newtonian space.
While rejecting substantival space, Berkeley does not deny regularities or the success of mathematical physics. He treats laws and forces as descriptions of observed regularities among sensible phenomena and probes how far metaphysical commitments are needed to ground those descriptions.

God and Causation
Berkeley's immaterialism colors his account of causation and the origin of motion. Bodies, on Berkeley's view, are collections of sensible qualities sustained in perception; God plays a central role as the continuous cause or preserver of orderly regularities. This theological dimension offers an account of the stability and intelligibility of motion without recourse to matter as a substratum endowed with hidden powers.
Berkeley stops short of reducing all physical explanation to divine intervention in a capricious way; God is presented as the sustaining mind whose regular will grounds the stability needed for scientific description. That move aims to reconcile the empirical success of mechanics with a metaphysics that eschews material substance.

Method and Argumentation
"De Motu" is written in the style of careful philosophical critique rather than mathematical elaboration. Berkeley attends to conceptual clarity, exposing inferential gaps and equivocations in the arguments for absolute space. He repeatedly urges that interpretation of physical practice should respect the limits of what is empirically given and warns against reifying theoretical posits beyond necessity.
The treatise engages Newtonian claims respectfully but firmly, deploying thought experiments, conceptual analysis, and appeals to the economy of ontological commitments. Berkeley's strategy is diagnostic: identify where metaphysical assumptions stray from what experience warrants and offer a more parsimonious conceptual framework.

Significance
Although less famous than Berkeley's writings on perception and idealism, "De Motu" occupies an important place in the early modern debate over space, motion, and the metaphysical commitments of natural philosophy. It anticipates later relational critiques of substantival space and contributes to the philosophical background against which successors would frame the debate about absolute versus relative motion. The treatise illustrates how metaphysical convictions about mind and existence can shape one's assessment of physical theory and remains a revealing episode in the history of philosophy of science.