Discussion

8.4 4 Inst Instance-unobs ance-unobserved erved

What about Instance-unobserved ? Instance-unobserved

An observed F is G justifies

All unobserved Fs are G.

The labors of chapters 5 and 6 were motivated by the thought that something like Instance-unobserved must be true. But we saw in the last chapter that such a principle needs to be restricted, and a similar dialectic plays out here.

Sneaker Raven

The ravens paradox in the form of the Wason selection task.

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N

Naattuurraal l kkiinnddss NNoot t nnaattuurraal l kkiinnddss

Quark Things owned by the Queen Atom People less than 4 ft tall Molecule White things

Acid Non-ravens

Emerald Things that tremble as if mad

Organism Things that resemble flies from a distance Raven Fabulous things

Human Sneakers

8.4.1 Natural Kinds 8.4.1 Natural Kinds

Quine (1969) argued that Instance-unobserved is only true when applied to natural kinds . Natural kinds are the type of thing that scientists investigate, and which cut reality at its joints. The best way to get a grip on them is with examples:

As ravens are natural kinds, we expect ravens to have the same properties, so an observed black raven suggests that unobserved ravens are black. By contrast, as white things are not natural kinds, we do not expect white things to have the same properties, so one observed white sneaker does not suggest that unobserved white things are sneakers. Pic torially, we expect everything in the shaded area to be in the same box.

Using natural kinds, we can strengthen Instance-unobserved from:

Instance-unobserved An observed F is G

We expect only one of these boxes to be occupied

We do not expect only one of these boxes to be occupie d

Black raven Black sneaker Black raven

White sneaker E White raven

White raven

THE PARADOX OF THE RAVENS

THE PARADOX OF THE RAVENS ¹²¹

justifies

All unobserved Fs are G.

to:

Instance-unobserved, Natural Kind That an observed F is G and F is a natural kind

justifies

all unobserved Fs are G.

As whiteness (F) is not a natural kind, an observed white (F) sneaker (G) fails to justify that all unobserved white things are sneakers.¹⁰

But it turns out that Instance-unobserved, Natural Kind is both too strong and too weak.

8.4.2 Objection to Natural Kinds: Too restrictive 8.4.2 Objection to Natural Kinds: Too restrictive

One objection is that it isn’t clear what a natural kind is. (This is analogous to the unclarity regarding what a projectible property is in 7.4.)

But even if it were, it seems too strong to say that Instance-unobserved is only true for natural kinds. That my observed pocket (F) contains francs (G) justifies that all my unobserved pockets (F) contain francs (G), even though my pockets are not a natural kind. So Instance-unobserved is not only true for natural kinds; Instance-unobserved, Natural Kind is too restrictive, telling us that there is no justification when in fact there is.

8.4.3 Objection to Natural Kinds: Not restrictive enough 8.4.3 Objection to Natural Kinds: Not restrictive enough

The following possible distribution of objects suggests that Instance- unobserved, Natural Kind isn’t restrictive enough.¹¹ Assume H and −H are the only possibilities.

–H

–H HH

B B

W W

Black raven Black sneaker

White raven White sneaker Black raven Black sneaker

White raven White sneaker

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Observing a black raven entails that you are in the possibility on the left, where there are ravens, including a white one. So an observed black raven refutes the hypothesis that all unobserved ravens are black. But ravens are a natural kind, so Instance-unobserved, Natural Kind wrongly says that the hypothesis is justified.

The following examples make a similar point.

The 99 ft man

Hypothesis: Unobserved men are less than 100 ft tall Instance: An observed 99 ft man

Additional details: Learning that a man could grow to 99 ft, you suddenly realize that a man could grow to 100 ft. So the hypothesis is “disjustified”

by the instance (where “disjustified” means that the probability falls, by analogy with “disconfirmed”).

The Grasshopper ¹²

Hypothesis: Unobserved grasshoppers live south of Canada

Instance: An observed grasshopper 3 inches south of the Canadian border Additional details: You previously thought grasshoppers could only survive in relatively mild climates. But if there are grasshoppers 3 inches from the Canadian border, there are almost certainly grasshoppers in Canada. So the hypothesis is disjustified.

In these cases, the distribution of properties means that the hypothesis is disjustified by the instance. Which constraints on the distribution of properties are necessary and sufficient for something like Instance-unobserved to be true remains an open question. (An application of Jackson’s approach to the new problem of induction (7.5) would suggest that the properties of the observed objects must be representative of the population.)

Whatever the general answer to this question, we can consider the separate question of whether there is justification given the actual distribution of ravens, sneakers, whiteness, and blackness. We’ll see that the hypothesis that unobserved ravens are black is justified, but only to a relatively small degree.