Sumários

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:

  1. What are thought experiments and how do they differ from scientific experiments?
  2. What is the epistemic role of thought experiments? (Are they sources of knowledge / justification?)
  3. If thought experiments are sources of knowledge, what is their proper domain? (What is it we can learn about from them?)
  4. Are thought experiments just disguised arguments, or something more?
  5. 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!

The existence of God

7 Outubro 2025, 11:00 • David Yates

This week we will examine two versions of the infamous ontological argument for the existence of God. We will begin with Anselm's classic version and consider how an atheist might respond. We will then consider Plantinga's modal version of the ontological argument, and again consider how an atheist might respond. We will then study the problem of evil for theists, and consider how theists might respond. The purpose of these classes is to see how philosophers with opposing views not only offer arguments for their own position, but also need to respond to their opponents' arguments. When you defend a position using an argument, you must respond not only to your opponent's objections to your argument, but also to their arguments for their own position, which you are committed to denying!

  • Suppose you argue for the existence of God and your opponent argues that there is no God. You can't both be right!
  • If your argument for the existence of God is sound, then your opponent's argument against God's existence is not sound.
  • Just because your conclusions are opposite, that doesn't mean either of you has an invalid argument! It's possible that both of you is arguing correctly from a logical point of view, but it's not possible that both arguments are sound.
  • Your opponent will not only offer a positive argument for the non-existence of God, they will also offer additional arguments either that: (i) your argument is invalid, or (ii) your argument contains at least one false premise.
  • You will also target your opponent's argument in this way! It all gets very complicated, very quickly. If nothing else, learning the philosophical method will teach you to be humble: are you really that sure of your own beliefs? If so, on what grounds?

Anselm's Ontological Argument

"Thus even the fool is convinced that something than which nothing greater can be conceived is in the understanding, since when he hears this, he understands it; and whatever is understood is in the understanding. And certainly that than which a greater cannot be conceived cannot be in the understanding alone. For if it is even in the understanding alone, it can be conceived to exist in reality also, which is greater. Thus if that than which a greater cannot be conceived is in the understanding alone, then that than which a greater cannot be conceived is itself that than which a greater can be conceived. But surely this cannot be. Thus without doubt something than which a greater cannot be conceived exists, both in the understanding and in reality." Proslogion II (tr. William Mann [1972: 260–1].

Here is one interpretation of this passage, adapted from the Internet Encyclopaedia of Philosophy Entry on Anselm and the Stanford Encyclopaedia Entry on Ontological Arguments (sections 6 and 11):

  1. We can understand the idea of God as a being greater than which no greater being can be imagined (that is, we understand the idea that God is by definition the greatest possible being that can be imagined).
  2. What we can understand exists as an idea in the mind.
  3. Therefore, God exists as an idea in the mind.
  4. A being that exists as an idea in the mind and in reality is, other things being equal, greater than a being that exists only as an idea in the mind.
  5. Thus, if God exists only as an idea in the mind, then we can imagine something that is greater than God (that is, a greatest possible being that does exist).
  6. But we cannot imagine something that is greater than God (for it is a contradiction to suppose that we can imagine a being greater than the greatest possible being that can be imagined.)
  7. Therefore, God exists.

This argument attempts to establish the existence of God from the very concept of God. If God is by definition that greater than which nothing can be conceived, then God exists by definition, since if He did not, we could conceive of an even greater being. What, if anything, is wrong with this argument?

Plantinga's Modal Argument

This argument is inspired by Anselm's and is very influential. Instead of arguing that God exists by definition, Plantinga argues that if it is possible that God exists, then it is necessary that God exists. In possible worlds speak, this means: If God exists at any possible world, then He exists at all possible worlds. But the actual world is one of the possible worlds, so if it is possible for God to exist, then God actually exists. Plantinga first defines maximal excellence and maximal greatness:

  1. A being is maximally excellent in a world W if and only if it is omnipotent, omniscient, and morally perfect in W; and
  2. A being is maximally great in a world W if and only if it is maximally excellent in every possible world.

He then argues as follows:

  1. The concept of a maximally great being is self-consistent.
  2. If 1, then there is at least one logically possible world in which a maximally great being exists.
  3. Therefore, there is at least one logically possible world in which a maximally great being exists.
  4. If a maximally great being exists in one logically possible world, it exists in every logically possible world.
  5. Therefore, a maximally great being (that is, God) exists in every logically possible world.

What, if anything is wrong with this argument? How should an atheist respond to it?


The Problem of Evil

Atheists often argue as follows (from Stanford Encyclopaedia of Philosophy):

  1. If God exists, then God is omnipotent, omniscient, and morally perfect.
  2. If God is omnipotent, then God has the power to eliminate all evil.
  3. If God is omniscient, then God knows when evil exists.
  4. If God is morally perfect, then God has the desire to eliminate all evil.
  5. Evil exists.
  6. If evil exists and God exists, then either God doesn’t have the power to eliminate all evil, or doesn’t know when evil exists, or doesn’t have the desire to eliminate all evil.
  7. Therefore, God doesn’t exist.

What, if anything, is wrong with this argument? How should a theist respond to it?

Para os que quiserem ler sobre o argumento de Anselm em Português, recomendo este artigo em linha: Critica na Rede, "O argumento Ontológico" (William Rowe, tr. Vitor Guerreiro). Este artigo aborda o argumento ontológico de Anselm.

Este artigo escrito pelo filosofo Desidério Murcho é mais difícil, e aborda o argumento ontológico de Plantinga: Critica na Rede, "Argumentos Ontológicos".

Vejam também: Critica na Rede, "O problema do mal"