One reason we "need" forms is to account for common-sense ideas we have about pigs.   For example, identity over time.  If a pig is merely a set of particles arranged pigwise, then when one subatomic leaves the pig swarm, it is not the same pig because it is not composed of the same set of elements.   Another common-sense idea is morality.   Why is it wrong to rip a pig or a dog in half but not a pencil?  If they're both just particles arranged in a certain way, and properties like life are just a higher-level illusions, then I don't see where that morality could come from.  A utilitarian would obviously disagree, but then again a utilitarian is unable to answer the question, why ought I be moral?  In fact the utilitarian doesn't even have access to the ideas that could answer that question.

These are not the only reason people posit forms, but the reductionist either has to accept some strange theories to account for these things or bite the bullet and say morality and personal identity aren't real.

Brian wrote:

One reason we "need" forms is to account for common-sense ideas we have about pigs.   For example, identity over time.  If a pig is merely a set of particles arranged pigwise, then when one subatomic leaves the pig swarm, it is not the same pig because it is not composed of the same set of elements.   Another common-sense idea is morality.   Why is it wrong to rip a pig or a dog in half but not a pencil?  If they're both just particles arranged in a certain way, and properties like life are just a higher-level illusions, then I don't see where that morality could come from.  A utilitarian would obviously disagree, but then again a utilitarian is unable to answer the question, why ought I be moral?  In fact the utilitarian doesn't even have access to the ideas that could answer that question.

These are not the only reason people posit forms, but the reductionist either has to accept some strange theories to account for these things or bite the bullet and say morality and personal identity aren't real.

Why can't we just say it's a set of particles arranged pigwise? This would solve the identity issue. The pig would be any set of particles which is arranged in a manner that we traditionally identify as being a pig. Even if it lost some particles and gained new ones, those new particles would become part of the pigwise arrangement and still seemingly be the same thing--like a car that has its tires replaced. We still call it the same car because it maintains a consistent arrangement of parts which we've tracked since our purchase of it.

As for the morality issue, well couldn't the reductionist concede that the macroscopic features and properties of a pig are ultimately illusory but still hold that it would be wrong to rip the pig in half insofar as the pig is someone else's property or that certain social standards have ingrained in him a sense of moral weight to his actions?

Drovot, citing the material parts of a given structure does not do a damn thing to explain away the structure or form. It is downright ridiculous to conclude that a pig is a "storm of quarks." Why? Because it obviously is not--there's an entire unifying macroscopic reality that is supported by this "storm of quarks." Give me a good reason to privileged the hidden subatomic structure over the macroscopic structure. Give me a good reason to conclude that the former is really real and the latter is merely an ephemeral veil, an epiphenomenal illusion. The pig is not reducible to these quarks for the very reason that, as you're so fond of using the term, we recognize it as a pig.

The form of the pig establishes the strictures that its parts must adhere to. There are no free-floating quarks with a pig--the powers of the quarks are limited, given impermissible and permissible behavior in accordance with the form that informs them. The pig is not reducible to its parts because its parts are fully yoked to the singular objective datum that we experience--the one and only pig. This is in stark contrast to an accidental form like a shovel--the iron for the head still behaves like iron, the wood still behaves like wood, the rubber grips still behave like rubber. In other words, the behavior (loosely speaking) of the shovel is reducible to its parts. With the pig this isn't so. Common sense requires us to view the pig and its inner structures from a top-down angle. We only understand why its organs are arranged the way they are, why its bones are arranged the way they are, its chemical, molecular, atomic structures are arranged the way they are, by reference to the whole, the pig. 

Drovot wrote:

Theoretically I could explain the pig as a pig by tracing the individual motions of each particle of matter in accordance with physical laws.

I also take issue with this.

Even if this could be proved, it does nothing to show that forms don't exist. Even if you could track each particle's motion and watch as they all--following certain physical laws--collapse into each other to form a specific structure, it still would not follow that what results from the physical motion of these particles is not an actually existing form we call a pig. What if I could track every particle from their primordial beginning in the core of a star to you? Would this mean that somehow you are not you, that you aren't of a specific kind? Do you think that, ideally, for you to exist as the kind of thing your are (rational animal) you needed to have popped into existence, your matter contrived out of nothing, not obeying or determined by physical laws? Do you think that forms need to unify matter by going against the laws of physics? This seems dubious. AT wouldn't deny that the material world is governed by physical laws--but it would deny that this somehow makes the notion of form obsolete. 

Drovot,

I think it is important to differentiate certain issues that aren't perhaps entirely distinguished in the OP. For a start, there is the distinction between the question of realism vs nominalism from that of the truth of the A-T conception of forms. That is, whether (at least some) universals and general terms, whether pig or chair or justice, exist independently (though not necessarily separately) from the particulars that instantiate them or whether all existing things are individual and general terms and universals are all just human made groupings is separate, though related, to whether A-T realism is true. The A-T concept of forms is an account of how universals might have a real existence, but it certainly isn't the only one. Arguments are given against the different accounts, but, generally, the main impetus for nominalism, in my view, is a reductionism that is weary of granting that something like an immaterial, abstract universal can exist. There are, however, some good arguments for realism. Deduction seems to require realism, for example. If we can't intuit that, for example, being mortal is necessary for all men, it is hard to see how the classic syllogism about Socrates' humanity and mortality could be anything more than an inductive (probable) argument based on past experience. And, obviously, there are many variations on appeals to our very ability to group individuals as belonging to a class (why do we call certain individual things pigs and other dogs?), which have already been brought up, or indeed how nominalism can account for the sharing of any properties at all.

​But as I said, none of this requires one endorse the A-T version of realism specifically. But I'm also not sure that claiming only sub-atomic particles and the like have substantial forms would be ruled by the A-T understanding of forms. Even if one denies that everyday objects are substances, the A-T philosopher can still claim that quarks et al. must be substances, given what he thinks are the objections to nominalism as well as competing realist accounts.