= Solution
<Newton's first law> says that an unforced body maintains uniform straight-line motion in an <inertial frame>. It can function partly as a criterion for identifying <Inertial frame>[inertial frames], but that does not automatically empty it of empirical content. The substantive claim is that there is a common class of frames in which independently isolated bodies have this behavior, rather than needing a different accelerated frame for each trajectory.
Testing requires auxiliary judgments about <Force>[forces] and measurement. If every unexpected <acceleration> is redescribed as an unknown <force>, or if a new <coordinate system> is tailored to every body, the law becomes insulated by construction. In practice <Force>[forces] are investigated independently through interactions, shielding, environmental changes and comparison among bodies. Reducing <drag (physics)>[drag] and varying external influences allow extrapolation toward the ideal. Mutually consistent force-free trajectories and the success of the broader <dynamical system> then support the inertial-frame hypothesis.
A simple counter to the claim that any coordinate choice suffices is a rotating laboratory: free bodies generally have velocity-dependent <Coriolis acceleration>[Coriolis accelerations] and cannot all be made uniformly rectilinear by calling that <rotating frame> inertial while retaining the same <force> account. Conversely, no finite experiment establishes complete isolation or exact uniform motion forever. The law is tested as part of a network of <Force law>[force laws], geometrical assumptions and <clock>/ruler procedures, and may have local rather than global validity in a gravitational theory.
The first law is therefore neither an independently observed slogan with no definitions nor a mere tautology. Its definition-like role fixes a useful framework, while its applicability and the consistency of that framework across systems are answerable to experience.<The Mathematical Principles of Natural Philosophy by Isaac Newton (1846 edition)>[Axioms or Laws of Motion, Law I and Corollary V]{ref}
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