unfalsifiable

Unfalsifiable refers to propositions or theories that cannot, in principle, be refuted by any possible observation or experiment. In the philosophy of science, the term is closely associated with Karl Popper's criterion of falsifiability as a demarcation between science and non-science: a theory is scientific only if it makes predictions that could, in principle, be tested and potentially shown false.

Some claims are unfalsifiable by nature (metaphysical statements, tautologies, logical certainties). Others are claims subject to ad hoc adjustments that shield them from refutation. Examples include certain religious or metaphysical assertions, some conspiracy theories, and theoretical frameworks whose empirical consequences have not yet been accessible or are underdetermined by data.

Criticism and nuance: Popper's criterion has been challenged by philosophers such as Imre Lakatos and Paul Feyerabend, who argued that science proceeds through evolving research programs and that falsification is not the sole measure of scientific value. Some domains, such as certain areas of cosmology or theoretical physics, may be currently unfalsifiable due to technological or theoretical limits, yet are pursued for their explanatory power. Conversely, some descriptions of reality may be unfalsifiable but regarded as metaphysical rather than scientific.

In practice, scientists strive to formulate testable predictions and to revise theories in light of evidence. Unfalsifiability thus signals limits on empirical refutation rather than the truth value of a claim, and it often marks the boundary between empirical science and philosophy or faith-based discourse.