worms because their behavior is less obviously analogous to our own, but
many people firmly believe that their behavior indicates that they also
have conscious experiences such as pain.
2.Physical similarity. Other people—and, to a lesser degree, various other
species of animals—are similar to me in their basic biological and physical
structure. Although no two people are exactly the same, humans are gen-
erally quite similar to each other in terms of their essential biological con-
stituents. We are all made of the same kind of flesh, blood, bone, and so
forth, and we have roughly the same kinds of sensory organs. Many other
animals also appear to be made of similar stuff, although they are mor-
phologically different to varying degrees. Such similarities and differences
may enter into our judgments of the likelihood that other creatures also
have conscious experiences.
Neither condition alone is sufficient for a convincing belief in the reality of
mental states in another creature. Behavioral similarity alone is insufficient be-
cause of the logical possibility ofautomatons: robots that are able to simulate
every aspect of human behavior but have no experiences whatsoever. We may
thin kthat such a machine acts as if it had conscious experiences, but it could
conceivably do so without actually having them. (Some theorists reject this
possibility, however [e.g., Dennett, 1991].) Physical similarity alone is insuffi-
cient because we do not believe that even another living person is having con-
scious experiences when they are comatose or in a dreamless sleep. Only the
two together are convincing. Even when both are present to a high degree,
I still have no guarantee that such an inference is warranted. I only know that
I myself have conscious experiences.
But what then is the status of the functionalist argument that an alien
creature based on silicon rather than carbon molecules would have mental
states like ours? This thought experiment is perhaps more convincing than the
electronic-brained automaton because we have presumed that the alien is at
least alive, albeit using some other physical mechanism to achieve this state of
being. But logically, it would surely be unprovable that such silicon people
would have mental states like ours, even if they acted very much the same and
appeared very similar to people. In fact, the argument for functionalism from
multiple realizability is no stronger than our intuitions that such creatures
would be conscious. The strength of such intuitions can (and does) vary widely
from one person to another.
The Inverted Spectrum Argument We have gotten rather far afield from visual
perception in all this tal kof robots, aliens, dogs, and worms having pains, but
the same kinds of issues arise for perception. One of the classic arguments re-
lated to the problem of other minds—called theinverted spectrum argument—
concerns the perceptual experience of color (Locke, 1690/1987). It goes like this:
Suppose you grant that I have visual awareness in some form that includes
differentiated experiences in response to different physical spectra of light (i.e.,
differentiated color perceptions). How can we know whether my color experi-
encesarethesameasyours?
The inverted spectrum argument refers to the possibility that my color expe-
riences are exactly like your own, except for being spectrally inverted. In its
12 Stephen E. Palmer