-Realism says abstract objects such as possible worlds are real and objective.
-Nominalism says abstract objects such as possible worlds aren’t real. Possibility must instead be grounded in the material world.
-Conceptualism says abstract objects such as possible worlds are real but exist only in the human mind. — AJJ

This isn't correct, really.

First of all, conceptualism is a species of nominalism.

But more importantly, one need not be a materialist to be a nominalist.

In fact, nominalists do not even necessarily reject abstract objects, although that is one popular form of nominalism.

On Googling conceptualism you get this philosophical definition: “the theory that universals can be said to exist, but only as concepts in the mind.” Then from the Wikipedia page: “Conceptualism is anti-realist about abstract objects”.

On Googling nominalism you get this philosophical definition: “the doctrine that universals or general ideas are mere names without any corresponding reality.“

Those seem to me to match the ones I’ve given from Feser’s book, which seem fine for the point being made. I understand you can be a nominalist about certain things while being a realist about others, but nominalism or conceptualism about possible worlds appear to have the implications I described.

Have a look at the SEP entry for nominalism:

https://plato.stanford.edu/entries/nominalism-metaphysics/

"Nominalism comes in at least two varieties. In one of them it is the rejection of abstract objects; in the other it is the rejection of universals . . . The two varieties of Nominalism are independent from each other and either can be consistently held without the other."

"Similarly, according to Concept Nominalism (or Conceptualism), there is nothing like scarletness and a thing is scarlet in virtue of its falling under the concept scarlet"

I'm a nominalist a la a conceptualist, by the way. I happen to both reject abstract objects and universals, and I'm a materialist, but it's not necessary to hold both types of nominalism or to be a materialist. Nonmateralist nominalists will simply hold that there are nonmaterial particulars.

Sure, I did a moment ago:

Nominalism comes in at least two varieties. In one of them it is the rejection of abstract objects; in the other it is the rejection of universals. Philosophers have often found it necessary to postulate either abstract objects or universals.

Like I said, I understand you can be nominalist about some things and realist about others. The definitions given in the OP are correct for the purpose of the point being made. I can at least concede that realism being true about possible worlds wouldn’t make it true about other abstract objects or universals, although it may as well be given the implications of that (the existence of a Platonic third realm or a divine intellect).

The definitions given in the OP are correct for the purpose of the point being made. — AJJ

The definitions, for example, say that nominalists are necessarily materialists. This is wrong.

So why are you saying it's correct?

The definitions, for example, say that nominalists are necessarily materialists. This is wrong. — Terrapin Station

Where does it say that?

"Nominalism says abstract objects such as possible worlds aren’t real. Possibility must instead be grounded in the material world."

It says no such thing as "possibility must be grounded in the material world."

That has nothing at all to do with nominalism.

It says no such thing as "possibility must be grounded in the material world." — Terrapin Station

Your objection was that the definitions say nominalists are necessarily materialists. Being nominalist about possible worlds doesn’t mean being a materialist, it just means being one about possible worlds since they’ve been rejected as existing in the abstract.

What part of "nominalists DO NOT say that possibility must be grounded in the material world" don't you understand?

That has nothing to do with nominalism.

What part of "nominalists DO NOT say that possibility must be grounded in the material world" don't you understand? — Terrapin Station

If possible worlds have been rejected as existing in the abstract then possibility must be grounded in the material world. So for example the possibility that it’s going to rain would instead be a description of the black clouds in the sky, say.

If possible worlds have been rejected as existing in the abstract then possibility must be grounded in the material world. — AJJ

No. This is wrong. I already explained the alternative. One can simply posit nonmaterial particulars. "Not abstract" doesn't imply "material." (And likewise, "abstract" doesn't imply "not material.")

Someone could be an ontological idealist (so reject materialism wholesale) AND be a nominalist.

I already explained the alternative. One can simply posit nonmaterial particulars. "Not abstract" doesn't imply "material." (And likewise, "abstract" doesn't imply "not material.") — Terrapin Station

If you edit your posts after I’ve replied to them I might not see the edit.

Can you tell me the difference between a nonmaterial particular and an abstract object such as a possible world?

(And likewise, "abstract" doesn't imply "not material.") — Terrapin Station

And I Googled abstract and this is the first definition given: “existing in thought or as an idea but not having a physical or concrete existence”.

Particulars are discrete existents, singular instantiations, with properties that uniquely obtain in that discrete instance.

Abstracts range over multiple instantiations of particulars, whether they're types/universals or concepts.

And I Googled abstract and this is the first definition given: “existing in thought or as an idea but not having a physical or concrete existence”. — AJJ

Which isn't correct, because you can have physical/concrete abstractions. For example, if you believe that abstracts are concepts, you believe that concepts are events in a specific individual's mind, and you're a physicalist on mind.

This is why we say, by the way, that nominalists about abstracts/abstractions reject that there are any real abstracts. ("Real" there amounts to "objective" or "external to mind.")

They do not necessarily reject abstracts as concepts. Hence we have conceptualist nominalists (which is what I am).

Abstracts range over multiple instantiations of particulars, whether they're types/universals or concepts. — Terrapin Station

I don’t see how this applies to possible worlds, which I take to be discrete abstract objects.

if you believe that abstracts are concepts, you believe that concepts are events in a specific individual's mind, and you're a physicalist on mind. — Terrapin Station

In that case concepts wouldn’t be abstract, rather they’d be concrete.

They do not necessarily reject abstracts as concepts. Hence we have conceptualist nominalists (which is what I am). — Terrapin Station

I’d say the definition in my OP recognises that.

I don’t see how this applies to possible worlds, which I take to be discreet abstract objects. — AJJ

There are a bunch of different metaphysical interpretations of what possible worlds are. You'd have to explain how "discrete abstract" makes sense to you (unless you're simply using "abstract" as a synonym for "nonphysical," but I explained why that doesn't work).

In that case concepts wouldn’t be abstract — AJJ

They're abstract in terms of content, or in terms of semantics (meaning). Content-wise, they range of a number of particulars. That's the whole function of concepts.

Re abstracts/abstraction, at least the beginning of the Wikipedia article on it is decent. I don't know if I agree with everything in the article, but the basic definitional idea is more or less on-target:


"Abstraction in its main sense is a conceptual process where general rules and concepts are derived from the usage and classification of specific examples, literal ("real" or "concrete") signifiers, first principles, or other methods.

"An abstraction" is the outcome of this process—a concept that acts as a common noun for all subordinate concepts, and connects any related concepts as a group, field, or category.[1]

"Conceptual abstractions may be formed by filtering the information content of a concept or an observable phenomenon, selecting only the aspects which are relevant for a particular subjectively valued purpose. For example, abstracting a leather soccer ball to the more general idea of a ball selects only the information on general ball attributes and behavior, excluding, but not eliminating, the other phenomenal and cognitive characteristics of that particular ball.[1] In a type–token distinction, a type (e.g., a 'ball') is more abstract than its tokens (e.g., 'that leather soccer ball'). "

https://en.wikipedia.org/wiki/Abstraction

You'd have to explain how "discrete abstract" makes sense to you (unless you're simply using "abstract" as a synonym for "nonphysical," but I explained why that doesn't work). — Terrapin Station

I’d say possible worlds are abstract according to the definition I quoted, and they’re discrete because they can be differentiated.

They're abstract in terms of content, or in terms of semantics (meaning). Content-wise, they range of a number of particulars. That's the whole function of concepts. — Terrapin Station

Well I’d want to make a distinction then between an “abstraction” and an “abstract object”. The former being applied more generally to concepts that may actually be physical in nature, and the latter applying to those objects which fit the definition of abstract I quoted.

Could you repost the definition of abstract you're using?

“existing in thought or as an idea but not having a physical or concrete existence”

“existing in thought or as an idea but not having a physical or concrete existence” — AJJ

So then, for one, in this context you'd be saying that possible worlds are objective thoughts or ideas? What would that amount to?

So then, for one, in this context you'd be saying that possible worlds are objective thoughts or ideas? What would that amount to? — Terrapin Station

It would amount to there being either a Platonic third realm where those objects exist, or a divine intellect where they do.

It would amount to there being either a Platonic third realm where those objects exist, or a divine intellect where they do. — AJJ

So we can't do possible worlds unless we buy platonism or god?

So we can't do possible worlds unless we buy platonism or god? — Terrapin Station

You can to an extent. But if you’re a nominalist about possible worlds then they must depend on the world around you and not be abstracted from it, which makes possible worlds such as one where the laws of physics are entirely different from ours impossible to have been.

If you’re a conceptualist then possible worlds can exist in the human mind, but then possible worlds where humans don’t exist become impossible, since if you rewound time to before the existence of humanity that possibility would disappear (as my own understanding has it).

I'm not sure that makes sense to me. Remember that I actually am a nominalist (about everything), and I'm the conceptualist brand of nominalist.

On my view, possible worlds are a way of talking about the simple fact that not everything about our world is strongly/causally deterministic.

Nominalists, by the way, if they reject abstracts, typically are not realists on physical laws, because it's difficult to construe physical laws as such as something other than abstracts. Physical laws as (real/objective) particulars don't make a heck of a lot of sense.

On my view, possible worlds are a way of talking about the simple fact that not everything about our world is strongly/causally deterministic. — Terrapin Station

So on your terms (as I understand them) a possible world would be a concrete abstraction in your brain, and so only a possibility so long as it existed there or in another person’s brain.

Possibilities are real--they're the fact(s) that the world isn't strongly/causally deterministic. That doesn't hinge on thought, but it's not abstract, either.

Possible world talk is a way of talking about the above fact(s).