Suppose you hold that the world consists of powers all the way down: all properties consist of causal powers. Now combine that with Hume’s position on our knowledge of powers: we have no impressions of powers, and hence no adequate conception of powers. Then you are committed to an extreme Kantian view of objective reality: the world is noumenal, because powers are. That is, we have no knowledge of reality as it exists outside the mind; at best we have a kind of structural knowledge of how things hang together, but no knowledge of the intrinsic nature of things. For that nature is essentially a matter of powers, and we cannot conceive of powers as they are in themselves; we are at most acquainted with mere signs of powers, ultimately what happens in our minds as they interact with reality. Thus the world of appearance is cut off from the world of reality: we seem to experience categorical observable properties of things, but objectively reality consists of unperceivable powers. Hume himself didn’t identify all properties with powers, but he did hold (in effect) that causal connections are noumenal; extending his epistemology to a wider metaphysics of properties-as-powers yields extreme Kantianism about reality as a whole. If the world consists of powers, and we can’t form an adequate conception of powers, then we can’t form an adequate conception of the world. We can’t form an adequate conception of what the world fundamentally is.
Two objections might be made to this sweeping metaphysics and accompanying epistemology. The first is that it is implausible to identify the world with a constellation of powers—there have to be grounding categorical properties somewhere in the picture, because powers cannot stand unsupported. Let us concede the point for the sake of argument: it doesn’t follow that we have adequate ideas of reality, since the grounding properties might be unknown to us. It suffices for the argument that all observable or known properties are powers: even if there are categorical properties at the bottom of reality (belonging, say, to some advanced physics), all the properties we know about consist in powers. So everything we know of the world turns out to be an elusive power—shape, size, color, etc. What we think of as ordinary properties are really powers whose inner nature we cannot discern.
The second objection is that what happens in our mind cannot consist of powers, because we do know what happens in our mind—we know, say, what pain is, and that it is occurring now. But the powers view of properties can be extended to mental properties by distinguishing appearance from reality: we don’t have an adequate conception of what pain is, but we do know how it appears. Mental properties have causal powers as much as physical properties do—they are individuated by their causal powers—but these powers may occasion only signs of themselves in what passes before consciousness. Pain itself might be noumenal—as is the self on the Kantian conception. The mind might be a hidden reality remote from introspection, made up of powers of which we have no adequate conception (if we follow Hume on our knowledge of power).
What is true is that appearances cannot be construed as presentations of properties if the (generalized) Hume-Kant view is right. For we do know the nature of appearances, so they cannot be constituted by powers that transcend our grasp; they cannot be collections of properties that present themselves to the mind. No property can be presented to the mind as it is, if all properties are really powers that are inherently inaccessible to the mind. Once we accept the elusiveness of powers, along with the doctrine that properties are powers, we are landed in the kind of epistemology adumbrated by Hume and Kant (Kant merely generalizing Hume). Thus the stakes are high; and idealism threatens. The ubiquity of powers leads to a Kantian view of our knowledge of reality once Hume’s critique is accepted. If powers are metaphysically basic, yet epistemologically elusive, we end up with an unknown reality. What is known is mere appearance from which the notion of property has been expunged (if it can be).
 The work of Sydney Shoemaker on properties is an instructive reference, but the view has a wider currency. Phenomenalism in effect holds that all material object facts consist in powers to produce sense experience, and behaviorism holds that mental facts are identical to powers to produce behavior. Generally, it is the idea that reality consists of potentials—of what would happen if. Reality is best captured by conditionals of a certain sort. Everything is the power to do something.