Modal argument
from 350 BCE Aristotle begins it; the modern form dates from the 1960s
Reasoning not about how things are but about how they must be or could have been, and drawing conclusions about the actual world from that.
- Active
- from 350 BCE
- Where
- Athens, later Princeton
- Status
- In use
- Tradition
- Forms of inference
What it is
A modal argument turns on necessity and possibility rather than on truth alone. It establishes that something must be the case, or could not have been otherwise, or could have been different — and draws a conclusion about the actual world from that.
The crucial and easily missed feature is that necessity and possibility are not degrees of certainty. They are claims about the world, not about the arguer's confidence, and confusing the two spoils every argument of this kind.
How it works
Establish a modal claim: that something is possible, or that it is necessary. Show what follows. Watch the scope of the modal operator with great care, because *necessarily, if P then Q* and *if P then necessarily Q* are different claims and swapping them is the standard fallacy.
Possibility claims are usually supported by conceivability, and the gap between what can be conceived and what is genuinely possible is where most of the argument lives.
The form of the inference
- It is possible that P, or it is necessary that P
- If P is possible or necessary, then Q
- Therefore Q, as a claim about how things actually are
The argument
Proposition The bridge design is inadequate even though it has never failed.
- The design brief requires the structure to withstand any load it could face in service, not merely those it has faced.
- A loading that has not occurred in eleven years is not thereby impossible; the flood of 1954 is within the recorded history of the catchment and would recur under conditions the catchment can still produce.
- The design was verified against the loads observed since construction, not against the loads the site can produce.
- So a possible loading exists against which the structure has not been shown to stand.
Therefore The design has not been shown to meet the brief, and its record of not failing is not evidence that it does.
The whole argument turns on a possibility, not on anything that has happened. This is where modal reasoning earns its keep in practical life — safety, insurance and liability are all about what could occur rather than what has.
What it is good for
Engineering safety, legal liability, and in philosophy the questions about essence, identity and mind that cannot be settled by looking. It is the only way to argue about what has not happened.
Where it fails
The move from conceivability to possibility is the standing weakness. I can conceive of water not being H2O in the sense that I can entertain the thought; it does not follow that it is possible, and Kripke's argument is that it is not. Every conceivability argument has to answer this and most do not.
Scope errors are the other failure and they are subtle enough to survive publication. *Necessarily, if he is sitting he is sitting* is trivially true; *if he is sitting, he is necessarily sitting* denies that he could have stood up. The medievals had names for the distinction and the tradition then lost it for centuries.
Common misconceptions
That necessary means certain and possible means uncertain. These are claims about the world, not about anyone's state of mind, and the whole method collapses if they are read as confidence levels.
Where it came from
Aristotle's modal syllogistic is the first attempt and is notoriously tangled. The medievals made real progress. The modern form dates from Kripke's semantics in the early 1960s, which made rigorous reasoning about possibility available for the first time.
What became of it
Transformed by possible-worlds semantics into a precise and heavily used tool, and central to metaphysics ever since.
Lasting influence
Naming and Necessity redirected analytic philosophy, and modal arguments now do much of the work in philosophy of mind, personal identity and philosophy of religion.
Why it still matters
High in philosophy, and quietly ubiquitous in engineering and law where it goes by other names.
Put another way
Testing a roof against the storm that has not come yet.
Texts
Saul Kripke, Naming and Necessity
Three lectures, delivered without notes, that changed the subject. Readable in a weekend and worth a month.
Where to start
Read Kripke. Then look for scope errors in ordinary argument; once seen they are everywhere.
The surprising bit
Aristotle's modal logic contains errors that stood uncorrected for two thousand years, and reconstructing what he actually meant remains an open scholarly problem.
In their own words
Necessity is the mother of invention is a plain misuse of a hard word
Who held it
the medieval modal logicians
Connections
- Influenced
- Possible worlds
- Parallel to
- The thought experiment
The verdict
Precise since the 1960s, and still resting on judgements about what is possible that nothing can check.