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4, a considerable variety of methods have been used in attempting to understand what quantum mechanics is really about. Some of these methods have used the existing mathematical frameworks as starting points, and have attempted to interpret it in ways that seem intelligible. Entities such as events, particles or waves are postulated to exist, and to have properties which are compatible with the existing rules of quantum mechanics. Their properties are rather strange from the common-sense point of view, but they have been seriously put forward as descriptions of what the quantum world is really like.
An alternative approach starts not from existing physical theories, but from some general considerations about what could possibly exist. This approach is less closely connected with specific physical theories, but does make extensive connections with the endeavours of philosophers over the centuries to consider `what exists' in a general sense.
These two different approaches are manifest in the career of A.N. Whitehead. To start with he was concerned with constructing an ontology of events, in order to make sense of Einstein's Special Theory of Relativity, and published books such as The Concept of Nature (Whitehead ) Later, however, he turned to a more philosophical approach, and attempted to frame what he called a `coherent, logical and necessary system of general ideas' in order to give a account within the philosophy of nature of what the world could be like. His Process and Reality (Whitehead ) of this period is therefore not closely related to the relativity and quantum theories which were concerning the physicists of his day, but provided a much more general framework of ideas by means of which specific physical theories could be formulated. His philosophy of nature in fact had less influence than he hoped, but physicists such as Stapp  have found them a useful starting point in the investigation of quantum processes.
What I am attempting here is to frame a coherent and logical system of general ideas in Whitehead's sense, but we do not end up with the same system as he did. The main difference can be attributed to the greater importance I give to dispositional properties and to potentialities.
Before we come to potentialities and dispositions, however, it is convenient to begin our investigations with a determination of what actually exists. Because we are sensitive to the problems of dispositions, etc., the next chapter will try to determine what can exist in a full and primary sense of actuality, a sense which excludes all potentialities and possibilities.
To know the whole character of a particular concrete thing in our changing world, is to know its nature. This describes both what sort of thing it is, and also the sources of any changes it may cause or undergo. The nature of a particular is itself independent of our knowing, for it is the real internal constitution in, and the source of real change of, that particular. Change is an essential part of the natures of the particulars which are considered in this book. These particulars are therefore physical or natural existents, as opposed, for example, to the unchanging formal entities of logic and mathematics (whatever they may turn out to be). The present philosophy of nature seeks a clear understanding of change and causality, and how these occur in natural particulars.
We must therefore call into question Hume's attitude to the problems of causality and change. His attitude is to allow problems of epistemology to dictate the kind of things which are thought to exist, but we are not forced to accept this dictation. It is possible to pursue a distinctly non-Humean course5.1, and investigate, in general terms, the natures of particulars and the powers therein for change. Parallel with our different approach, we must reject Hume's ontology in which there exist only momentary events with no intelligible causal connections. It is also necessary, as will be discussed in the next chapter, to avoid commitment to any account of the relation between formal logic and set theory which denies the possibility of real change for particulars.
The way we get around Hume's problems is to adopt a hypothetico-deductive approach. That is, we hypothesise the characters of certain entities which could exist, and follow out deductively the consequences of that hypothesis. Only later do we consider what might have these characters, or indeed whether anything at all has the characters we hypothesised. There are therefore deductive and necessary connections within the theory being proposed, but only empirical and contingent connections of the theory with the real world. This follows exactly the `conjectural essentialism' scheme that in section 2.6 was found most satisfactory to describe natural dispositions and physical laws.
This means that the arguments in the next two chapters are rather general, and should be valid whatever in the physical world in facts turns out to be actual and potential. It is only in chapter 8 that specific instantiations are proposed for some of the general concepts. It is perhaps thereby rather frustrating to the impatient reader, who finds it difficult to imagine to himself exactly what is going on, but similar problems arise whenever mathematical or logical proofs are being presented. In a sense, we are here developing a `process logic' which should have a number of distinct fields of application as well as our target application in the interpretation of quantum physics.
I will in fact exploit the uncertainty of application in the order of
arguments in the following chapters. Chapters
(and those following) will assume a specific identification of `what
is actual', namely as past events at points in space-time.
This, however, will turn out to be not that best suited for the
world described by quantum physics. That identification, however, provides
a simple and easily-understood model for a kind of quantum
world, so I will use it to illustrate and clarify the basic concepts at
issue. Only in chapter
will this (contingent) identification be replaced by another, namely that
what is actual are past selections at points in a tree-like structure of
alternatives. Since, though, the arguments in our `process logic'
have general validity, we can carry over many of our previous conclusions
to the more complete model.
Next: 6. Actuality Up: Philosophy of Nature and Previous: 4.4 The Problem of Prof Ian Thompson