The problem of other minds is the philosophical problem of determining how we know that there are minds associated with the bodies we see walking around among us. This is more specifically a problem of epistemology.

The problem can more accurately be expressed by breaking it into several steps.

1. We cannot know that when you look at a red object you have the same perception of red that I do (i.e. We cannot know that if I were to inherit your state of mind at that time, I would not call the sensation green)
2. Therefore, we cannot know that other people have any sensations at all as opposed to automatic nervous reaction.
3. We can never directly know another's mental state.

Conclusion: We can never know that there exist any other minds but our own. This can lead to the philosophical position known as solipsism.

In response to this problem there have been different areas of attack.

The reductionist viewpoint, supported by John McDowell and others, has tried to tackle the first two propositions 1 and 1a, by putting forth certain modes of expression (such as being in pain) as privileged and allowing us direct access to the other's mind. Thus, although they would admit from the problem of pretense, that at no one time can we claim to have access to another's mental state, they are not permanently unavailable to us.

Counter to the reductionist argument would be a more biological theory (and somewhat materialistic viewpoint). Take the eye and the perception of color. The light-sensing cone cells of the retina that respond to the portion of the spectrum designated as "red" are tuned similarly in every person tested, so we might expect all people to experience red in the same way. However, we also know that some people are missing certain (or all of) types of cone cells in the eye; thus giving rise to color blindness and other such visual variances. Similarly, differences in the distribution of brain cells and dendritic connections (among many other potential variances) could give rise to different mental states for the same stimulus. Cross-culturally, when people have a word for red, they agree with other cultures on which wavelengths of light best fit the term "red" (the same wavelengths that primarily excite the cone cells which detect red, and the red/green channel to the brain). Yet even if human eyes and brains may be built in such a way that the same wavelengths stand out for everybody, still it is conceivable that for different individuals these wavelengths could evoke experiences that differ. In particular, one external stimulus may give different experiences to the same individual according to which eye is used.

Lastly, some people see the very fact that we can discuss the problem of other minds and relate with one another on this existential angst as proof against solipsism.

See also

• color perception
• psychophysics
• qualia