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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 [1] 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.

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At the macroscopic level, we have a conception that things occupy space in a distinct and definite way. Things have boundaries, and two things cannot occupy the same space at the same time, i.e. they are not interpenetrable. But at the microscopic level, we have learned that this picture substantially breaks down.

Atoms are not like billiard balls: most of their volume consists of shells of electrons that are diffuse in nature and are often referred to as clouds. These clouds have a kind of shape to them, but not one with distinct edges, but rather one that is described by a probability density function that is calculated from a wave equation. When atoms combine into molecules, these electronic clouds overlap and form complex patterns that are also described by wave functions, but there is no point at which one can say that one atom ends and another begins. As a result, it is difficult to define exactly what the radius of an atom is.

The same kind of picture emerges when we dig deeper and examine subatomic particles. In fact, the picture becomes even more weird. For example, there are particles called neutrinos which, because they do not experience the strong nuclear force nor the electromagnetic force, can pass straight through other matter with very little probability of interaction.

Now if we ask, what is it that is occupying space? the answer sounds rather vague: it is a story of fields and waves and potentials described by equations that yield only probabilities. To take up Blackburn's use of "ifs", we used to think simply, if this object is here then another object cannot be here as well, but now it becomes something more complex like, if something is here and something else is here their wave functions will combine in a way that is described by this equation.

The reason all this is relevant to philosophy is that one of the classic philosophical questions is: what kinds of things exist? If the classical idea of distinct things or particles is only a macroscopic approximation, does this mean that only fields exist? Blackburn seems to think we hanker for something more.

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