Science developed as a synergy of concerns, with an ever-shifting balance between idealism and realism (or materialism). The realist thread in physics focuses on the role of the external world in supplying empirical data and driving their interpretation. The thread of idealism in physics emphasizes the role of the theorist in creating theory and in defining, collecting, selecting, and interpreting data. Pushed to the extreme, scientific idealism is deductionism—the belief that the world can be reduced to a logical system, an idea. The empiricism and ontological materialism of science necessarily rest upon a bedrock of idealism, occasionally exposed in the upheavals of scientific advance. This is evident, for example, in the hope of a final theory: the insistence by some theorists that physical phenomena can be exhaustively and definitively mapped mathematically in theory. However, this faith that reality can be reduced to deductive systems implies that it is not conceived as material at all, but as ultimately ideal. For example, the classical “realism” espoused by Einstein in opposition to the “incompleteness” of quantum theory is not realist at all. Rather, it insists that physical reality should correspond perfectly to human definition. In that way, it is guaranteed to be comprehensible, with an identifiable cause for every effect. Despite Einstein’s secularism, this axiom of completeness harks back to the religious heritage of science, which viewed material reality as a product of divine definition by a rational mind, whose role is to be taken over in modernity by the theorist’s definition. (This suggests that some of Einstein’s aphorisms invoking God might be more than mere figures of speech.) Religion is essentially idealist in outlook. While idealism is not in itself religious, its versions in science tend to side with religion against the autonomy and immanent reality of nature. But if nature is real, no theory can be complete.