 Note, too, that the category of explanatory empiricism suggests a particular view of scientific explanation. It suggests that the causal factors to which an explanation appeals will be hidden aspect of the natural world, facts which lie “beneath,” as it were, the observable reality of things. An explanatory empiricist denies we can know such facts. Aristotle is cautious regarding our ability to know about matters that cannot be directly observed. As he writes, when it comes to “matters inaccessible to the senses,” it is sufficient if our investigation arrives at a possible cause (Meteor. 344a5–7). But Aristotle would also deny that we need knowledge of what is unobservable in order to have a satisfactory explanation. The causes invoked by an Aristotelian science are not occult or hidden. On the contrary, the explanatory properties of things are open to view. It follows that the arguments that constitute a scientific explanation are not inferences from an observable fact to an unobservable cause, such as the behavior of Democritean atoms. They are demonstrative arguments referring to active powers whose operations can be directly perceived. Even when a particular event (such as the formation of hailstones) cannot be directly perceived, a plausible explanation will be one that appeals to observable phenomena (such as the way objects are made smooth by friction). One apparent exception is found in Aristotle’s explanation of the movement of the outermost heavenly sphere. This posits a different kind of explanatory principle: a divine substance or being (ousia) which is “eternal and unmoved and separated from perceptible things” (Met. 1073a4–5). For the most part, however, Aristotle’s scientific explanations invoke perceptible realities.