We distinguish problems and mysteries: questions that we can in principle answer and questions that exceed our cognitive capacities. It is natural to interpret this distinction ontologically: some things are mysterious while others are merely problematic. The world divides up into entities that are mysterious and entities that are merely unknown. Thus it may be supposed that mind is mysterious but matter is not. But this may be underestimating the pervasiveness of mystery: perhaps everything is mysterious—everything physical, everything mental, and everything abstract. To be sure, there exist mere problems, many of which have been solved or will be solved, but these relate to aspects of things not the thing itself. For example, there are problems of calculating the motions of bodies that can be solved, but the origins of motion and the nature of what moves may be irresoluble mysteries. Perhaps nothing is totally mysterious, but then nothing is totally non-mysterious either. Everything presents both problems and mysteries. Some aspects of consciousness are not mysterious, such as the kinds of conscious state there are, but there are also mysterious aspects of consciousness, such as its relation to the brain. Accordingly, science (the human kind) might apply to some aspects of everything but not to all aspects. Newton’s theory of gravity combined these two features: he gave us a science of motion with predictive mathematical laws but he left the origin and basis of motion a mystery—gravitational force is thus both intelligible and unintelligible. Might this be the general state of things—understanding combined with incomprehension? A single thing has both mysterious and transparent aspects; it is not that some things are mysterious and other things transparent. The two properties are intertwined not exclusive. We live in a generally mysterious world but certain aspects of it yield to our understanding. That is presumably the position of other animals: they can solve many problems but the world is generally mysterious to them. Hume thought that all causation is mysterious, and causation is everywhere, so everything has a mysterious aspect—but other aspects of things are intelligible to the human mind. Physics provides intelligible theories (more or less) but it doesn’t plumb the mysteries of matter; it limits itself to certain aspects of matter. Mathematics presents an ideal of intelligibility but the nature of numbers remains obscure. Psychology enables us to understand each other (to some degree) but its constructs are baffling. We have only a partial understanding of everything we understand: some aspects of things remain mysterious. At any rate, we should distinguish this view from the view that some things in nature are mysterious while other things merely present soluble problems. Mystery might thus be universal but not total. To put it differently: for any problem there is a correlated mystery. 
 These brief remarks are not intended to persuade anyone of the mysterian position; they are only for the initiated. There are many interesting questions about the nature and types of natural mystery and about its extent once one has become persuaded of the general truth of the position.