Digital twins are too often described as realistic simulations, anatomical avatars, dashboards, or data mirrors. Those artifacts can be useful, but they miss the defining property of a digital twin: bidirectional feedback between a physical counterpart and a virtual counterpart. The physical system continuously updates the virtual one; the virtual system informs actions that change measurement, intervention, operation, or governance in the physical world. We propose such a bidirectional feedback as the organizing principle for digital twins and apply it to a nested, multi-scale hierarchy of biological and social organization, in which lower-level units combine into higher-level systems, producing desirable properties at each level, from cells and tissues to organs, individuals, organizations, and population at large. Neuroinformatics is a stress test for this view because brain health, dementia, epilepsy, and other neurological diseases require the integration of cells, circuits, behavior, care pathways, and the translation of discovery to practice. Examples from epilepsy care and consortium-scale brain-cell atlas production show that digital twinning is not merely multi-scale modeling. It is a rich, multidisciplinary paradigm of computing for designing, governing, and driving feedback loops that turn data into accountable action.

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