maintained this in the seventeenth century. Might he have
been dead wrong? Could it be that all animals, and even
plants--and even bacteria--have minds?

Or, to swing to the other extreme, are we so sure that all
human beings have minds? Maybe (to take the most extreme
case of all) you're the only mind in the universe; maybe
everything else, including the apparent author of this book,
is a mere mindless machine. This strange idea first occurred
to me when I was a young child, and perhaps it did to you as
well. Roughly a third of my students claim that they, too,
invented it on their own and mulled it over when they were
children. They are often amused to learn that it's such a
common philosophical hypothesis that it has a name--solip-
sism (from Latin for "myself alone"). Nobody ever takes
solipsism seriously for long, as far as we know, but it does
raise an important challenge: if we know that solipsism is
silly--if we know that there are other minds--how do we
know?

What kinds of minds are there? And how do we know?
The first question is about what exists--about ontology, in
philosophical parlance; the second question is about our
knowledge--about epistemology. The goal of this book is not
to answer these two questions once and for all, but rather to
show why these questions have to be answered together.
Philosophers often warn against confusing ontological ques-
tions with epistemological questions. What exists is one
thing, they say, and what we can know about it is something
else. There may be things that are completely unknowable to
us, so we must be careful not to treat the limits of our knowl-
edge as sure guides to the limits of what there is. I agree that
this is good general advice, but I will argue that we already
know enough about minds to know that one of the things
that makes them different from everything else in the uni-
verse is the way we know about them. For instance, you
know you have a mind and you know you have a brain, but

-2-