I think we might separate the ideas that there's only one type of particle and that the world is reductionist. It is an open question as to whether everything can be reduced to a single fundamental thing (like strings) and it wouldn't be a logical impossibility to discover that there were two or three kinds of things interacting. (Or would it?)

Reductionism, as I understand it, is the idea that the higher levels are completely explained by (are completely determined by) the lower levels. Any fundamentally new type of particle found would just be added to what we consider "lower level".

So what does it say about the world that it is reductionist? I propose the following two things are being asserted:

(1) There's no rule that operates at an intermediate level that doesn't also operate on the lower levels. This means that you can't start adding new rules when a certain level of organization is reached. For example, if you have a law that objects with mass behave a certain way, you can't apply it to everything that has mass but not quarks. This is a consistency rule.

(2) Any rule that applies to an intermediate level is reducible to rules that can be expressed with and applied at the lower level. For example, we have the rule that two competing organisms cannot coexist in the same niche. Even though it would be very difficult to demonstrate, a reductionist worldview argues that in principle this rule can be derived from the rules we already apply to quarks.

When people argue about reductionism, they are usually arguing about (2). They have some idea that at a certain level of organization, new rules can come into play that simply aren't expressible in the lower levels -- they're totally new rules.

Here's a thought experiment about an apple that helped me sort through these ideas:

Suppose that I have two objects, one in my right hand and one in my left hand. The one in my left hand is an apple. The one in my right hand has exactly the same quarks in exactly the same states. But somehow, for some reason, they're different. This implies that there is some degree of freedom between the lower level and the higher level. Now it follows that this free state is determined in some way; to determine an apple in my left hand and a non-apple in my right, either by some kind of rule or randomly, or both. In any case, we would observe this rule. Call it X. So the higher level, the object being an apple or non-apple, depends upon the lower levels and X.

(a) Was X there all along ? If so, X is part of the lower level and we just discovered it, we need to add it in to our lower level theory.

(b) What if X wasn't "there" all along? What if for some reason, X only applies at intermediate levels? ...either because

       (i) X is inconsistently applied or because 

       (ii) X is not describable as a  function of lower level terms

The case (a) doesn't assert anything about the universe, it just illustrates a confusion that can result from not understanding what "lower level" means. I don't think (b) in either part is logically impossible because you can run a simulation with these rules.

Until you require (and obviously you want to) that the universe is a closed system. Then I don't think you can have b(i) or b(ii). A rule (Rule 1) that is inconsistently applied (bi) requires another rule (Rule 2) determining when to apply it. Rule 1 being inconsistent in a system means that Rule 2 is outside the system. If a phenomenon cannot be described by the states of the system (the lower level) (bii) then it depends on something else outside the system. So I think I've deduced that the logical impossibility of reductionism depends upon the universe being a closed system.

If the physical universe isn't closed -- if we allow the metaphysical -- then non-reductionism is not logically impossible.

Where does randomness come in? Is the universe necessarily deterministic because of (bii) being impossible, so that the higher levels must depend deterministically on the lower levels? (I'm talking about whether a truly stochastic component is possible in Brownian motion or the creation of particles in a vacuum, etc).

Another thing to think about is how these ideas affect our theories about gravity. We have no direct evidence that gravity satisfies consistency or that it is expressible in terms of lowest level physics. Does anyone know if any well-considered theories are ever proposed for gravity that don't satisfy these rules?