Discussion Questions on van Fraassen


1.  Suppose two elastic bodies travelling along the same line collide.  Let the initial and final velocities of the first body be vi and vf and the initial and final velocities of the second body be Vi and Vf, and let the two masses be m and M.  The law of conservation of momentum tells us that

(a) mvi + MVi = mvf + MVf,

and the law of conservation of energy tells us that

(b) mvi2 + MVi2 = mvf2 + MVf2.

If we add a specification of the values of the masses and initial velocities, we can explain the final velocities.  That is to say, if we add equations:

(c) m = m*

(d) M = M*

(e) vi = vi*

(f) Vi = Vi*

Then we can derive values for vf and Vf.   (Substituting the quantities specified in (c) - (f) into (a) and (b), we would have two equations in two unknowns.)  Most people would regard this derivation as explaining why the two final velocities have the values that they do.  But one could instead specify the values of one of the final velocities and solve for the other final velocity and for one of the masses or for one of the initial velocities (whichever specification was replaced by the specification of one of the final velocities).  It is not clear whether we would regard a derivation of one final velocity, from the other final velocity, the masses, and one of the initial velocities as an explanation.  It is clear that we would not regard a derivation of the initial velocities as explanatory, and it is even clearer that we would not regard the derivation of the masses as explanatory.  (Indeed if somebody were to ask, "Why is the mass of this body M?" we might be inclined to reject the question.)

How would van Fraassen make sense of this?  Please remember this example, because later on I'll want to refer to it in discussing Kitcher's views.

2.  What does van Fraassen take explanatory power to be?

3.  Suppose someone investigating a forest fire were to conclude that the fire occurred because there was oxygen present in the atmosphere.  This conclusion would be regarded as absurd, a bad joke, even though everyone recognizes that oxygen does play a crucial role in fires.  How does van Frassen explain why citing the presence of oxygen fails to explain why the fire occurred?

4.  How might van Frassen respond to Salmon and Kitcher's criticisms?  Is there some way that he could reject their demand that he specify what count as relevance relations?