Metaphysics
The most natural way to move from causation to modality is to talk about determinism: the first section is therefore on determinism as a preface and preliminary. It is rather about why modality matters.
Regarding determinism we find that it is as old as philosophy; if we think of the elleatic philosophers we remember their teaching against change. Either something is or it is not, and there is nothing in between. If this is the case then everything is determined: it cannot be otherwise.
Today we generally speak of causal determination however which is a different story: we talk about change today, but rather in terms of that change having been predestined.
Necessitarianism is more closely the term that refers to elleatic philosophy.
Determinism: everything that is the case is causally determined to be so by antecedent causes, like previous states of affairs. This gives us the idea of a causal chain and necessitation by causes. This causal determinism is extremely popular in modern thinking. In Spinoza for example, everything that is the case is necessarily the case, which is ultimately grounded in the divine essence, identical with nature.
There is also a similar view in Schopenhauers, in which free will is a representation of our imagination, whereas in reality everything is causally determined.
We also have Laplace, who claimed that if there was an omniscient demon that knew all the natural laws, then that demon would be able to predict every future condition of any given entity at any given time. In the absence of God, the world is taken to be a kind of perpetuum mobile that keeps itself moving. Laplace answers Napoleon that for most assumptions he does not need God.
Leibniz and Wolff argue that ultimately free will can be reconciled with determinism if we mean that the causal mechanism is rational choice.
In the 20th century Frankfurt argues that free will does not entail the notion of choice. A will can be called free even in the absence of choices.
This is the background in which modal logic was developed.
Modal logic is very old. You find it already in Aristotle’s second analytic. He talks about possibility, impossibility etc. Aristotle systematised this. And as much did medieval logic. They thought a lot about predetermination as a logical notion. Basically we are confronted with the notions of the possible and the necessary.
DiamondP means that P is possible. Not-Necessary-not-P.
SquareP means that P is necessary. Not-Possible-Not-P
One can define these terms in relation to each other.
Every equivalence can also be seen as a definition; ie you can define possibility with the negation of necessity, and vice versa.
People generally start from some notion that they presuppose and then define another notion in terms of the first. This is a common strategy in modern logic.
P-And-Possible-P: contingency.
Not-Possible-P-And-Non-P: excluded midle.
Necessary-P-Then-P: Ps being the case is entailed by the necessity of P, which also entails its possibility: P-Then-Possibility-P.
In the 20th Quine was quite important. He was a logical empiricist, and frequently warned his students against modal logic, because ultimately it is trivial. But it is a temptation to find it non-trivial. For Quine, Logical empiricsism is a kind of actualism, it is only interested in what is actually the case, and not at all in possibilities. If something is the case, then it is possible. We can trivially implement the notion of possibility by looking at what is the case. Possibility as such however is not accessible to us: only actual things are observable. Possibilities are non-observable entities so they are suspicious to empiricists.
There is a Greek guy called Diadorus Chronus, who argued that there are no possibilites, only trivial ones. This became a famous critique of Aristotle, which he ultimately rejects in the 9th book of metaphysics.
Quine refers to this and argues that Aristotle has a misguided obsession with metaphysics which is why he rejects his view.
If we have an ordinary material implication, p→q, we can infer that if P is the case, then necessarily Q is the case. So we can trivially introduce necessity.
Everything relevant in philosophy can be articulated with first-order logic to Quine: modal logic is quite useless.
For Kripke, if natural laws are contingent, they can be otherwise.
The problem is analyticity, which seems to entail necessity, and then you cannot do without modal logic. Quine’s answer is to deny the distinction between necessity and analyticity, and argues that it is context dependent. What can be necessary in some context can be analytic in another context. It depends on convention. Bachelors are unmarried purely because that is the way in which we use the term. The quinean position is basically the bulward against metaphysics as such. If you give relevance to modal logic then metaphysics is back on the menu, and this is what Kripke does, by forcing a conversation on more demanding account of modality.
Another application of modal logic is legal and political theory’s deontic logic, which is developed by Von Wright – a student of Wittgenstein and one of his compilers after his death.
Kripke was taught as a logical empiricist but breaks with that tradition in thinking about modality. What Kripke perceives is a strange tension between the dogmas of logical empiricsm, and the committment to scientism. A view that is somehow science-friendly and science-supportive.
Science rotates around the notion of a natural law. A law is a modal notion, there is a kind of necessity in it. We can say that it is just an irregularity, but this is not an uncontroversial view in science itself. If we take science seriously we should also take the language of science seriously – especially then the notion of a law.
There are laws of nature which are not analytic, most of them in fact. They are genuine discoveries we make, and not definitions. To infer that they are contingent because they are a posteriori makes absolutely no sense. We have to look for something more robust, that is, using those logical means that Quine hates.
Leibniz of course also thinks using modal logic when he describes possible worlds, and saying that we live in the best possible world. God has chosen the best possible world, because it is perfect.
Kripke is not interested in the theological dimension, but rather a rigorous articulation of the idea of a possible world. Kripke has a rigorous calculus, with a notion of a possible world. This suggests that there is not just one world but several that we can compare within a certain scope.
Technically speaking a possible world is just a set of propositions that do not contain a contradiction, within itself. The definition of possibility is that everything is possible insofar as it doesn’t entail a contradiction.
| W: P, Q, V, S is a possible world.
| W2: P, -Q, V, S is also a possible world.
→it is necessary that P, V, S
→it is possible that Q
Alvin Plantinga offered a new variety of the ontological argument by trying to argue that if God is possible, this means that he exists in one possible world. If he exists in one possible world He must exist in all possible worlds, which means that His existence is necessary.
The core idea is that this argument is coherent. He doesn’t think it is a proof but rather a coherent claim.
Lewis wrote a famous paper critiquing Plantinga on this point.
Kripke argues that if we compare possible worlds, they are hardly populated by the same entities. But insofar as possible worlds have the same objects, they can be compared. Suppose that Nixon would’ve lost the election in 68, then we see that being president of the united states is not a necessary, or essential, to Richard Nixon. Kripke spells out this intuition by classifying Nixon as a rigid designator which only refers to one person in all possible worlds. In all the possible worlds in which he exists in that way, the Richard Nixon we refer to is the same.
Lewis assumes that the possible world in which Nixon does not contain Nixon, but a counterpart Nixon. So therefore you can only compare various counterparts of Nixon.
Kripke argues that ultimately the modalities are not free-floating, but are grounded in the essence of things.
Ideally a proper name works like a rigid designator, and we use it to trace individual substances, but it is not always the case that the proper name refers to the rigid designator.
Kripke developed his views together with Hilary Putnam, who was a kind of pragmatist. They shared views.
Kripke’s claim is in saying that the terms we use in science are used as rigid designators. So the water we speak of in common language is not actual water, it just looks like water. The water that science speaks of, H2O, is the rigid designator that holds throughout all possible worlds. It is left to the sciences to find what the set of essential properties is, the essential identification which seperates one thing from the other. The essence of a thing is taken to be a rigid designator that is present in every possible world.
If psychology and psychiatry differs on the definition of a depression, then perhaps this classification actually lacks an essence, it may just be an umbrella term or a family resemblance.
Designators are rigid IFF they capture the essence of things: this is how essentialism comes back to philosophy.
There are other interpretations of modal metaphysics.
David Lewis argues that all possible worlds are actual. From the point of view of another possible world, we are also a possible world.
One thing that Kripke does not pay attention to is the role of tense. His use of possibilities is atemporal, and abstract givens. It is an abstract structure, and his calculus is not finegrained enough to say something about possibilities that once were available and are not available anymore.
Here comes time-branched modal logic, which has several authors. The basic idea is that we can distribute possibilities along a timeline.
We have a moment t1 at which P is the case. P is compatible with Q and -Q, temporally speaking. Q and -Q are accessible at that moment in time. At t2 a change must have been made, which chooses one and excludes the other. And so on and so on.
We can thereby some tensed modalities, in which we can say that Q and -Q are equally possible at t1 but not at t2, where eg Q is actual and -Q is excluded.
This says something about possibilities and necessities at given moments. If there is only one way for history to move forward, then that is a necessary movement. An interesting aspect of modal notions is that they are time-related such that some possibility is given at some time but not in others. This does not really apply to natural laws, which are supposed to be necessary.
We can also use this framework of time to discuss models of determinism and indeterminism. Indeterminism is the claim that all of these possibilities are real possibilities at any given moment, whilst determinism says that these possibilities are an illusion.
This is then later a model for deontic logic.
Kit Fine argues that every real possibility is the possibility of an actual entity. Actualism. It goes against both Kripke and Lewis. Ultimately you must, in order to find out what is possible, find out what the given possibilities of a thing are. In the abstract you can say that it is possible that someone jumps 10 meters high. Kit argues that this is ridiculous. It is literally impossible for a person to jump 10 meters high in reality: we must look at how things actually are.
People would then argue that Fine is essentialism friendly. But we should rather look at powers and dispoistions. What is really possible with a given entity depends on the powers and dispositions that entity has. This is what Barbara Vetter argues. She thinks that modalities need to be grounded in powers and dispositions. They are basically part of what defnes the thing in question’s essence. In that sense, a living being has certain abilities that an inanimate object lacks. This is a further step away from empiricism, because empiricism is very skeptical about powers and dispositions which are entirely unobservable entities, and therefore obscure. The logical positivists found much issue with talking about a having and a predication. If we have sugar, that’s not some property of sugar that we see. Neither can we easily speak of sugar as soluble, because that’s literally invisible. Carnap argues that we simply don’t know dispositional properties, and we need to be skeptical about this because it is a gateway drug to metaphysics.
Many modal metaphysicians after Kripke argue that thought experiments are the method of philosophical progress. Thought experiments have a value of their own, and are a privileged way of getting metaphysical insight; which is now criticised as armchair philosophy.