Sumários

Discussion

24 Outubro 2025, 11:00 • David Yates

In the Friday session we discussed the Ship of Theseus puzzle, and fission cases, and their implications for the identity of both objects and persons over time.

One standard response to the ship of Theseus is to say that the ship at sea is the original, but composed of different parts; while the ship in the museum is a new ship, composed of the original parts of Theseus' ship.
On this theory, things are constituted by (but not identical to) specific collections of parts, which can change over time without the things in question losing their identity. So far so good...
But what then should we say about persons? If we are constituted of our fundamental physical parts, but not identical to them, it seems there must be two things located where we are at any given time: a person that can survive part replacement, and a specific aggregate (collection, structure) of fundamental physical particles that cannot survive part replacement.
Problem: at any given time, that specific collection of parts has all the same physical and mental properties you have at that time, so there seem to be two things where you are, thinking your thoughts. It's the same in the ship case: there is a specific collection of planks and a ship, but they can't be identical, because the ship can survive replacement of its planks, but the specific collection of planks cannot!
Maybe this is not much of a problem: the specific collection of fundamental particles located where you are at a given time t does not even survive one second later than t, since the particles are constantly in flux. Perhaps it does not even have time to think anything before it is replaced by a new collection!
The important point is that unless you think there is some essential metaphysical difference between ships and persons (and there may well be) then whatever you say about the ship, you should also say about persons and their identity. Can we survive replacement of our parts? Does it matter how many parts we have and how quickly they are replaced? Are you really numerically identical to a baby, or was Heraclitus right - can you ever speak to the same person twice?

We then discussed fission cases, which are cases where we imagine a single person dividing into two persons, either because of hypothetically possible brain surgery or a sci-fi transporter accident. These cases are often taken to suppose a four-dimensionalist conception of space a time, in which all times are equally real, just as all places are equally real, and we as human being are located at multiple times (that is, we have temporal parts). Fission cases, that is cases in which a single person seems to divide into two persons, are actually cases in which one 4D person has a strange Y-shaped structure. Nothing divides! Instead, the 4D person is the thing that has two (or more) branches when seen as a being that extends in space as well as time. On this view, when fission happens (e.g. your brain is copied and placed in a new body, or a transporter accident occurs) a Y-shaped spacetime worm starts off having only one temporal part that exists at any given time, as we all are now. But after fission, that same Y-shaped person later has two temporal parts that exist at the same time, and which have different thoughts and experiences. On 4D-ism, this is no more mysterious than the fact that you have two legs that exist at different places.

We will return to this discussion next week.

Thought experiments

21 Outubro 2025, 11:00 • David Yates

This week we will begin discussing thought experiments.

Reading suggestions

Chapter 3 of Chris Daly's book An Introduction to Philosophical Methods will be very useful for this topic. Also very useful will be Peg Tittle's book What if? Collected thought experiments in philosophy. I will refer you to specific thought experiments from Tittle's book in the topic descriptions below.

We will first introduce some metaphilosophical ideas, concerning the following questions:

What are thought experiments and how do they differ from scientific experiments?
What is the epistemic role of thought experiments? (Are they sources of knowledge / justification?)
If thought experiments are sources of knowledge, what is their proper domain? (What is it we can learn about from them?)
Are thought experiments just disguised arguments, or something more?
Do thought experiments depend on our intuitions, and if so, is that a problem?

Once we have briefly discussed some of these issues, we will begin our survey of some classic philosophical thought experiments with the famous Ship of Theseus puzzle. We will consider what, if anything, this imagined puzzle tells us about the nature of material objects and the way in which they persist over time. Briefly, the Ship of Theseus puzzle goes like this. Sailors at sea would repair their ships while at sea, and this often included the replacement of one or two planks of wood. A ship can clearly survice replacement of a single plank, that is, it is the same ship after the replacement as it was before. This is just the same as the way in which we humans survive replacement of our constituent molecules. But eventually, that means it is possible for a ship to contain none of the original planks that constituted it. This is an interesting fact, but not yet the puzzle we will be concerned with. Here is the puzzle:

Suppose the original planks and all the other parts of the original ship are kept after they have all been replaced. They are taken away to a museum, one by one, where they are eventually reassmbled to form a ship. Where now is the original Ship of Theseus? Is it the one that is still sailing at sea, that was gradually repaired, or is it the reassembled ship in the museum? Both ships, it seems, have a very strong claim to being the original. But originally, it seems there was only one ship, and now there are two! One ship cannot be identical to two ships, so where is the original Ship of Theseus? Things we will think about in class:

What does this puzzle and similar cases tell us about survival and identity?
Do the same lessons apply to us as people?
Is this similar to "fission cases" - another kind of thought experiment in which one person seems to become two - or is it different?
Were there two ships all along, which were eventually separated from one another?

Once we have considered these questions, we will return to our initial questions about the nature of thought experiments. Does the puzzle teach us anything we did not already know? If so, is it a disguised argument, or did we really conduct some kind of experiment into the nature of reality?

You will find brief descriptions of many, many thought experiments (including the Ship of Theseus and the others we will look at in this part of the course) in Peg Tittle's excellent book, "What if - Collected Thought Experiments in Philosophy"

Em português, este problema chama-se o Problema do Návio de Teseu. Este artigo no Compêndio em Linha em capaz de ser util - a primeira parte é relativamente fácil ler e contém uma versão simples do problema. https://compendioemlinha.letras.ulisboa.pt/wp-content/uploads/2015/01/composicao-material.pdf

In Friday's class we will consider Fission cases and Personal Identity over Time. Fission cases are thought experiments that depend, typically, on some empirical knowledge about how the brain works. However, they go further that that and ask us to imagine scenarios in which one person can seemingly divide into two.

Readings for this week (all very short, 1-2 page articles from Peg Tittle's book, describing some very interesting philosophical thought experiments):

Identity of material objects: "Hobbes' Ship of Theseus" (pp. 68-9)

Personal Identity: "Perry's Divided Self", pp. 82-3; and Parfit's Fission (pp. 84-5)

Test 1 - argumentation

17 Outubro 2025, 11:00 • David Yates

Written test on argumentation

Frewill and the consequence argument

14 Outubro 2025, 11:00 • David Yates

Determinism is the thesis that everything that happens in the physical world is fully determined by the past and the laws of nature. That means physical movement you make with your body is fully determined by events that happened before you were born. Every physical event has a fully sufficient physical cause, and causes necessitate their effects according to causal laws. Everything that happens in the future must happen, given the past and the laws.

What determinism is not: a thesis about what some sort of Laplacean super-being might be able to predict. Determinism has nothing to do with prediction!
In metaphysical terms, determinism is about how many possible futures there are, from the point of view of any given monent in the history of our world (/universe). According to determinism, there is only one possible future, from the perspective of any particular moment in time. (Also: there is only one possible past., but that need not concern us.)

It might seem intuitively obvious that if determinism is true, then we do not have freewill. This position is known as incompatibilism, because it is the claim that freewill and determinism are incompatible.

Against this, it has often been argued that even if determinism is true, our actions are still caused by our mental states. For example, you are reading this text because you wanted to, which is to say, your wanting to read the text caused you to read it. That remains true even if reading this text is the only thing you could possibly have been doing right now! And what could be more free than doing what you want, because you want to? This position is known as compatibilism, because it is the claim that even if determinism is true, we can still have freewill because our mental states still cause our actions.

This is where Peter van Inwagen's consequence argument comes in. He offers a deductive argument against compatibilism, based on the idea that freedom requires that things are under our control, that our actions are up to us. Based on some very plausible principles, van Inwagen argues that if determinism is true, then nothing we do is up to us. He concludes that if determinism is true, we do not have freewill. This argument has been very influential and many continue to find it persuasive. See what you think!

What should a compatibilist say in response to van Inwagen's argument? Does it have a false premise? Perhaps it is invalid? Notice how compatibilists and incompatibilists have different conceptions of freewill: for compatibilists, doing what you want, because you want to; for incompatiblists, choosing between different possible futures, each of which was open at the time of choice. Which of these conceptions is closer to your own personal understanding of freedom? If it is the second one, do you accept the conclusion of van Inwagen's argument? If you do, you must say either (1) we have no freewill, or (2) determinism is false!

Este artigo é uma introdução às noções centrais do debate, sem abordar diretamente o argumento de van Inwagen: https://criticanarede.com/hkahanelivre-arbitriodeterminismo.html.

For a brief explanation and a simple version of van Inwagen's argument (plus a more complicated version) see this 2-page article from the book Just the Arguments: van Inwagen's Consequence Argument against Compatibilism.

What kind of principle is premise P3 in the first version of the argument that you see in the above document? (Clue: you have seen principles of this kind before!)

Discussion on God and the problem of evil

10 Outubro 2025, 11:00 • David Yates

The two ontological arguments depend on conceptions of God that can be resisted (rejecting for example the idea that God is a necessary being, if He exists). If God is not maximally great, then we cannot use His greatness to argue for His existence.
The two ontological arguments also depend on a controversial claim about the role of imagination in modal epistemology, simply: if we can imagine something, then it is possible. Both arguments depend on the idea that the concept of God, as a maximally great being, is self-consistent, and both rely on imagination to establish this. But does this establish genuine metaphysical possibility, or does it only establish the weaker epistemic possibility ("for all I know, God is possible")? The Goldbach conjecture case from Murcho's article in Critica na Rede shows that if we use epistemic possibility in Plantinga's argument, we can easily establish a contradiction. Thus, that argument depends on imagination to establish that God is a real, metaphysical possibility (this is much stronger than the claim that, for all I know, God is possible).
The problem of evil can be resisted in at least two ways: (i) by claiming that a certain amount of "evil" is necessary for God to create the best of all possible worlds. This is what the freewill defence effectively says - God had to create a world with freewill, but we can use our freewill to commit acts of evil. Other defences along these lines suggest either that God allows us to suffer so that we can show virtues such as courage, without which the world would be morally less good than it is; or simply that what look to be evils to us may not look that way to God, who sees the entire cosmos at once. Perhaps in reality there is no true evil in our world. One problem for these responses is that it seems very difficult to imagine why e.g. child abuse or animal suffering could possibly be part of God's plan, or could possibly make the world a better place. The problem of natural evil is perhaps the hardest one to deal with: why are there earthquakes, floods and hurricanes? Nobody seems to choose these events, so their existence cannot be explained via the freewill defence. But perhaps these are cases of events that are good for the world as a whole (or the cosmos as a whole) without being good for us human beings.
Alternatively, (ii): we can revise our conception of God so that he is no longer all of omniscient, omnipotent, and benevolent. On this view, God may lack the desire to prevent suffering because he is not morally perfect; or he may lack the power to prevent all suffering, because he is not omnipotent. If we do this, however, we can no longer argue for God's existence based on our conception of Him as maximally great, because according to this response to the problem of evil, God is not maximally great!