ity around us violates the principles... which is why experimental
psychologists often use cheap magic tricks to produce counterintu-
itive situations.^2
Intuitive physics uses observable phenomena (like the motion of
objects) to infer what is intrinsically invisible. Consider for instance
causal connections between events. If you see a billiard ball hitting
another one, you can't but perceive that the second ball moved because
it was hit by the first one. Indeed, we sometimes think we "see" such
events even when we know that no physical object is involved. If you
show people colored discs moving on a screen, you can make them
"perceive" that one disc "hit" another one, "pushed" it along, and so [99]
on. This happens even if people know that what's on the screen are
just dots of light, so that there is nothing that is hitting or pushing
anything. Now if you adjust the displays a bit, you can make the
"causal illusion" disappear. What people see now are discs moving,
other discs moving too, but no "cause" and no "effect." In the 1940s,
psychologists Albert Michotte and Fritz Heider showed that such
"causal illusions" are very reliable—everybody reports that they "saw"
a cause—and that they depend on precise mathematical relations
between the motions of the objects—you can eliminate the illusion or
re-create it by adjusting the displays according to precise formulae.
More surprising, Heider and Michotte had also shown that dots on a
screen can make people believe that what they see are not just solid
objects moving and colliding but animate beings "chasing" or "avoid-
ing" each other. Again, by carefully adjusting the timing and spatial
characteristics of the dots' relative motion, you can create the illusion
of "social causation." In the same way as in the simpler experiments,
the adult subjects know that what they see are in fact just dots on a
screen. But they cannot help perceivethem as engaged in a "chase" or
as "trying to get somewhere."^3
In the previous chapter I gave a very simplified account of what it is
to have ontological categories. I suggested that we have a mental cata-
logue of the kinds of things that are around us, containing entries like
"animal," "person" and "man-made object," and a little theory about
each entry. The theory specifies for instance that animals are born of
animals of the same species, that the structure of man-made objects is
related to their use, etc. But the term theorymay be a bit misleading, so
here is a more precise description. Seeing or otherwise perceiving an
object activates a particular set of inference systems. Not all objects
activate them all. The fact that a certain type of object activates a cer-
THEKIND OF MINDITTAKES