FYI, in a thread on the old forum (https://groups.google.com/g/metaphysica ... 6U9eS6BAAJ
), BK said that his argument against solipsism was in his "Why Materialism is Baloney" book. Here are the relevant sections from that book regarding solipsism (the italics are BK's):
"Materialism requires the following four statements about reality to be true :
1. Your conscious perceptions exist;
2. The conscious perceptions of other living entities, different from your own, also exist;
3. There are things that exist independently of, and outside, conscious perception;
4. Things that exist independently of, and outside, conscious perception generate conscious perception.
....[irrelevant sections omitted]...
Let us look more carefully at a point that was already briefly mentioned before: the erroneous notion
that idealism somehow entails what is called ‘solipsism’ in philosophy.
Solipsism is the notion that all that exists are my own conscious experiences. In other
words, reality is purely my
private dream. There are no other conscious entities, like other
conscious people. They are merely figments of my own imagination. If I were a solipsist, I
wouldn’t believe that you, dear reader, have inner life at all. I would believe simply that your
external appearance and behavior, as far as I can perceive them, are imagined by my own
Now, notice that solipsism entails the acknowledgement of statement 1 of the previous
section and the rejection of statements 2, 3, and 4. Therefore, it is not idealism
grants reality to statement 2. ‘Why?’ I hear you ask. After all, if we are already following this
road of radical skepticism anyway, why grant reality to statement 2? Because believing in
statement 2 is the simplest explanation for observations.
As discussed earlier, I can explain
much of my own external behavior to myself by the fact that I am conscious, and so can
you. It is your conscious feelings that explain your facial expressions, your impulsive reac-
tions, your dislike of certain people and your love for others, etc. And you undoubtedly ob-
serve very similar external behaviors in others: their facial expressions, impulsive reactions,
likes and dislikes, etc. To explain these behaviors of others while assuming that others are
not conscious – that is, by assuming solipsism – would require an entirely different expla-
nation for largely the same phenomena that you observe in yourself. In other words, very
similar observations would require very different explanations. Clearly, this isn’t the sim-
plest alternative. It is simpler and more elegant to infer that others are also
manifest their external behavior for the exact same reasons that you manifest yours, partic-
ularly given the fact that others have physical bodies entirely analogous to yours.
You could argue that other people’s behavior is so analogous to your own because you
your conscious life onto them, in the same way that the characters of your nightly
dreams all have human-like reactions while being merely projections of your mind. This
way, other people would still be characters of your private solipsist dream, behaving like
you do simply because your ‘subconscious’ mind is projecting your own patterns of behav-
ior onto them. This sounds reasonable, doesn’t it? There is, however, a hole in this argu-
ment. Indeed, more than likely you have observed many types of behavior in other people
that you could not yet explain based on your own experiences, because at the time you
made those observations you had not yet had the experiences that would explain such
behaviors. For instance, as a young child, I could already observe the peculiar behavior of
adults in love without ever having had that feeling myself. I just couldn’t figure out why peo-
ple would act like fools in those situations. Later in life, as I experienced romantic love my-
self, I could immediately match that new personal experience to prior observations of the
‘foolish’ behavior of others and explain them retroactively by granting consciousness to
those other people. As a young child, I couldn’t have projected onto others an experience I had
not yet had.
Therefore, once again, it is simplest and most reasonable to accept statement 2
of the previous section. Idealism is very reasonable and skeptic, but it differs from solip-
sism in that the latter seems to be unreasonably skeptic."