Source: Think: A Compelling Introduction to Philosophy (1 ed, 1999) by Simon Blackburn
[p 249:] But if, following Faraday, we resolve particles themselves into yet further powers, dispositions, or forces, we cannot be satisfied with this kind of image. We have to try to understand what the cosmos contains without the mental crutch afforded by "things" of any kind whatsoever. Hume's complaint about impenetrability -- that we need to know what it is that cannot penetrate what -- then returns to haunt us.
[1.] It is as if the commonsense conception of the difference between space occupied by a body, and space not so occupied, has been displaced in favour of space of which some ifs are true, as opposed to space of which other kinds of ifs are true. But we hanker after something to really occupy space, whose presence explains the differences in ifs, the differences in potentials and powers.
Sorry for asking this if it is impertinent; would someone please simplify and explain the sentences in 1, where the ifs referenced are too vague? I understand the sentences before  and those below.
[p 250:] This is a problem that greatly exercised Kant, himself one of the pioneers of the resolution of matter itself into "forces". Kant thought that this conception of things was the best we could ever achieve. He thought this partly because we know of the world by means of the senses, and the senses are essentially receptive. That is, all they ever give us are the results of powers and forces. The senses are not adapted to tell us what in the world underlies the distribution of powers and forces in space. They simply bring to us the result of that distribution. Anything underlying it would have to be entirely "noumenal" -- lying behind the range of scientific investigation, and for that matter beyond the range of human experience and thought.
Hume thought that his problem with impenetrability cast doubt on the whole metaphysics of "the modern philosophy", although he also thinks Berkeley's own retreat into subjective idealism is entirely unbelievable. Kant too believed that the problem required an entire rethink of the modern philosophy.