In my previous thread about the web of reality, I argued that particular occasions of experience are the most fundamentally concrete parts of the world, and everything else that we postulate the existence of, including things as elementary as matter, is some abstraction that's only real inasmuch as postulating its existence helps explain the particular occasions of experience that we have.

Some of these abstract things are so fundamental that we could scarcely conceive of any intelligent beings comprehending reality without the use of them. Immanuel Kant called these kinds of things, things we cannot exactly observe but which we cannot help but use to structure the things that we do observe, "categories".

The ones that I will describe here are not exactly the ones that he describes, though there is significant overlap.

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The first thing we need to do to structure our experiences is to identify patterns in them. To do that, we need a pair of concepts that I call "quality" and "quantity", which allow us to think of there being several things that are nevertheless the same, without them being just one thing: they can be qualitatively the same, while being quantitatively different.

Any two electrons, for instance, are identical inasmuch as they are indistinguishable from each other, because every electron is alike, but they are nevertheless two separate electrons, not one electron.

In contrast, the fictional character Clark Kent is, in his fictional universe, identical to the character of Superman in a quantitative way, not just a qualitative way: though they seem vastly different to casual observers, they are in fact the same single person.

If two people are said to drive "the same car", there are two things that that might mean: it could mean that they drive qualitatively identical cars (or as close to it as realistically possible, e.g. the same year, make, and model), or it could mean that they drive the same, single, quantitatively identical car, one car shared between both of them.

With these concepts of quality and quantity, we can describe patterns in our experience as quantitatively different instances or tokens of qualitatively the same tropes or types. Out of this arise the notion of several different things being members of the same set of things ("qualities" as I mean them here mapping roughly to the mathematical concept of "classes", an abstraction away from sets, and "quantities" as I mean them here mapping roughly to the mathematical concept of "cardinality", an abstraction away from the measure of a set or class). And with that can be conducted all of the construction of increasingly complex abstract objects built from sets as detailed in another previous thread.

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Then to further structure those patterns, we need conceptual spaces in which to figuratively lay out those instances of those tropes, in which to cluster them together and separate them apart. The most elementary of those conceptual spaces, I hold, is what mathematicians and physicists call a phase space or configuration space, which is an entirely abstract, imaginary kind of space wherein each point represents one way the system under consideration could be, a kind of abstract space of possibilities, wherein the potential changes of our experiences can be structured.

If we then identify patterns, trends, in the movement of our experience through that abstract space of possibilities, we have constructed the concept of time in its usual linear sense, with one direction in the configuration space being designated the past and another designated the future. I hold that time is best conceived of literally as a line through an abstract configuration space like this, with other "possible worlds", other possible configurations of the world, being literally, ontologically the same kind of thing as other times, other times being merely a special subset of other possible worlds in which the present, or the actual world, can be found.

(My conception of possible worlds, each being an instantaneous possible configuration of the universe, is different from the usual kind promoted by supporters of modal realism — this view that other possible worlds really exist — like David Lewis, who hold that other possible worlds each contain within them a whole temporal history changing from past through present to future. Other philosophers, such as logician Saul Kripke, seem to take possible worlds to be instantaneous configurations of the universe like I do, and speaking of things being possible, necessary, etc, "from" one world or another, rather than in absolute terms; something being possible in some world if it occurs in any other world "accessible" from that world, rather than just if it occurs in any possible world at all. I would interpret that, on my model, as being equivalent to the temporal relationship between possible worlds: a world that is "accessible" from another is a possible future of that other world, and things that lie in possible futures of a given world are in a sense relatively possible from that world, but we can still speak meaningfully about things being possible in the absolute sense of occurring in any possible world at all, irrelative to any particular world from which that world is accessible).

With a concept of time established, we can then construct a concept of space, that being the time that it takes a change to one part of our experience to affect another part of our experience. This does not depend on any particular claims of contemporary physics about the speed of light being constant or anything like that: any arbitrary speed could be picked, even if the speed of light were not constant, by which to derive distances from durations, and to construct a concept of space from a concept of time. This is actually quite commonly done in casual speech: places may be said to be hours away by foot or minutes away by car, their distances given as the time it takes to travel there at a given speed (implied by the mode of transit).

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Within these spaces, we can then separate bundles of experiences from each other into different objects, giving rise to concepts such as substance, which is not directly observable and cannot really be said to exist unto itself, but is a useful concept for structuring bundles of experiences in space. Patterns in our experience bundled together in time likewise give rise to the concept of causation, which David Hume famously argued also cannot be directly observed, only patterns of "constant conjunction".

I agree with Hume about that as much as I agree with the likes of George Berkeley about substance, in that neither can really be directly known or said to really exist in any independent way, but they are nevertheless indispensable concepts in structuring the patterns of our experiences in space and time. (These notions of substance and causation also map neatly onto the notions of mass and energy, inasmuch as mass can be thought of as the amount of substance in an object, and energy thought of as the capacity to cause changes).

Lastly, the concept of a kind of modal identity is useful for structuring ideas about counterfactual scenarios, bundling things together across possible worlds in a way broader than mere temporal causation. For instance we might want to say that had I made different choices in the past I would find myself in different circumstances in the present, and yet that counterfactual me in some other possible present is still nevertheless me in some sense, just as much as the past version of me that we both have in common is also me in some sense, connected to this present me by a chain of causation. I am presently what that past me became, in this timeline; and some other possible me is what that past me could have become, had things unfolded differently.

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Two other useful concepts for structuring our experiences are what we might call mind and will, in very particular senses of those words, but I will explore those concepts in much greater detail in later threads. In short for now, the concept of "mind" as I mean it for these purposes is just that of whatever it is that is the subject of experiences, as opposed to the objects of those experiences: minds in this sense are whatever it is that are having the experiences, in contrast with whatever it is that the experiences are of. As discussed in the previous thread (same link at the top of this post), I hold that these are not ontologically different kinds of things, but rather, different roles that the same things can take on, and that all things do take on simultaneously.

Note that by "experience" here I don't necessarily mean anything so robust as the kind of experience humans have, though that is a subset of what I do mean, which is something much more simple, shared by all objects; similar to, and I think identifiable with, what is meant by "observation" in quantum mechanics, which does not require a thinking person, but just any other physical system capable of interacting with the thing being "observed". (I will detail my thoughts on "mind" in a more robust and useful sense than this in my later thread on that topic).

And the concept of "will" as I mean it for these purposes is likewise a very simple subjective role that I hold anything can and everything does take on (with my thoughts on "will" in a more robust and useful sense than that are likewise to be detailed in my later thread on that topic). But rather than being the subject of experience, "will" as I mean it here is the subject of behavior, which is to say it is essentially agency in a very broad sense, the capacity to behave, to do things, in the same way that I mean "mind" here as the capacity to experience.

And experience and behavior, as I elaborate in the thread linked at the start of this post, I hold to be but two different perspectives on the same singular thing: interaction.