Idealist and skeptical traditions have long held the world to be somehow illusory. Medieval Christianity denied the immanent reality of nature by holding it to be a created artifact. The Enlightenment continued this tradition by discounting natural reality in favor of conceptual schemes. Ever since Descartes, the modern imagination has embraced the notion that the appearance of reality can be deliberately counterfeited. Aside from observer-independence, however, another symptom of nature's reality is its essential resistance to formalization. Realism is generally at home in classical physics, where imprecision was not taken as indeterminism, much less to mean that the world has no definite existence or properties. The resistance of inertia, nature's resistance to simplifying thought is an essential aspect of its autonomy. Anything made may be expressed in terms of propositional knowledge. Such an expression may represent something in the way that a mental image does but not in the way that an optical image does.