Mereology is the study of parts and wholes. The name comes from the Greek word meros, for “part.”
Here are some questions and proposals debated under this heading:
Is the whole physical universe an individual thing? Or is it only a plurality or collection of other things? If there’s more to reality than just the physical universe — perhaps souls exist too? — then is there any individual thing that includes all of them plus the physical universe?
Some philosophers believe that (despite appearances) there are no individual things. For example, some physicists may say that what look like elementary particles to us are really modifications or wrinkles in a more fundamental field, instead, and count the field as a stuff rather than an individual thing.
Some philosophers believe that (despite appearances) there is just one individual thing. Some say this thing is God. Others say it’s something like the enormous things mentioned in point #1 — except that these philosophers insist it’s the only thing that exists. According to them, smaller things like our planet and bodies and the particles that make us up wouldn’t also exist, as parts of the enormous thing. At any rate, they wouldn’t exist as individual things. These philosophers may say those things are instead something like different wrinkles in the enormous thing. (More on this idea when we talk about substances.) Or they may say they’re just different ways in which the enormous thing “presents itself.” (In this case, all the allegedly “smaller” things would really be identical to the enormous thing.) Or they may say the smaller things are just illusions.
Some philosophers have thought they had good reasons for saying these things. Their views may be worth thinking through and discussing, but I hope they’ll seem far-out and implausible to you. We ought to hear some really compelling arguments before thinking any view like these may be right.
A less far-out, but still surprising, view is that many individual things exist, but all of the ones that do are simple particles. Larger things composed out of many particles — like the whole physical universe and our planet and bodies — none of these really exist, as individual things. There are merely pluralities or collections of particles arranged in certain planet-like or body-like shapes. These views are more popular among philosophers than the views described in points #2 or #3, but they too are rather counter-intuitive. We ought to hear some compelling arguments before thinking these views may be right, either.
The most common-sense view is that particles exist as individual things, and also some larger individuals like planets and bodies exist, too. The larger individuals are composites that have other smaller individuals as parts. On this common-sense view, though, not just any plurality or collection of individual things makes up a further individual. (The “wholes of reality” mentioned in #1 might or might not be among the larger individuals that really exist.) For example, a random subpart of the particles in a table, and a random subpart of the particles of my body, don’t together make up any further individual thing.
A more inclusive view accepts all the individual things that the common-sense view #5 does, but abandons their hang-ups about the weird collections. Sure, they say, there’s an individual thing that includes just the particles described (and no more). It’s just that we’re not normally interested in objects like that; and so don’t have names for them.
Another debate in mereology arises for views like those in points #5 and #6, who think that at least sometimes, there are individual things that have other individuals as parts. The question is: When there are such composite indiduals, how many can be made up of a given plurality or collection of parts at one time? For example, some particles make up the wood of a table, and also make up the table itself. Is that wood the same thing as that table? Some philosophers argue they are one and the same thing. Other philosophers argue they are different. Other philosophers argue only one of them really exists as an individual thing.
Recall our earlier discussion of churches and a nightclub that succeed each other in inhabiting a building. What are the relations between one of the churches and that building? Assume that the church doesn’t have any further buildings or property. Then: should we say it’s identical to its one building? Or to a stage in that building’s existence? Or is the church some different object, that merely inhabits or is located at that building?
Our aim in introducing these debates now isn’t to try to figure them out. That would be very challenging to try to do at the start of one’s engagement with philosophy! Also, even many people who’ve been doing philosophy a long time find these debates “weird” and don’t see how to make progress on them. They’re not even sure if the different positions we’ve described all represent really different options.
All we’re trying to do at this point is to get a feel for the kinds of options philosophers consider here, and what assumptions do they think we might not be able to take for granted or rely on without argument.
Our course is not going to be a study of mereology or ontology in full generality. We are going to be focusing on the special question of what the identity conditions for persons are.
Sometimes the arguments and issues we discuss will be ones that also come up for other objects, too, like ships or statues or planets. Other times they won’t. Some of the more general questions, we’re just going to help ourselves to assumptions about.
For example, we’re going to assume (along with points #5 and #6 above) that such larger things as human brains and bodies exist.
Also, we’ll be thinking about cases where we have to make assumptions about whether a given brain or body has remained numerically the same thing over time. Brains and bodies, like other large physical objects, are gradually losing molecules and having them replaced, as time goes by. So in principle there could be Brain of Theseus type cases. There are interesting questions about what makes brains and bodies preserve their identity over time. But we’re not going to be able to look into those questions. We can’t do everything at once! We’ll just have to assume the cases we’re considering are ones where numerically the same brain or body does continue to exist, in the way we say it does.