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The same would go for the existence of the entire world. If you accept introspectively known sense impressions as valid information, you realize that you have no other information. All your sensory data are known to you and only you, by mental impressions. A touch on the arm is known as the mental feeling of a touch on the arm. The arm itself knows nothing. All you can know for sure is the mental impressions you have of the world. You can’t know if anything else is really “out there.”
In the most extreme form, a solipsist asserts, “I am the only self that exists. All the rest of the world is, at best, a hypothesis, or possibly just a figment of my imagination.”
There is no way to refute solipsism. Any counter-argument against it would just be another figment of my imagination. If it is false, I could never know it, because my own mind is the only thing known to me. Solipsism is an extreme form of idealism, which says that only mental events can be known to exist (or, only mental events do exist).
Consequently, in scientific psychology, introspection is not allowed. No introspective observations can be accepted into discussion of how the mind works because introspection is private, and if you accept private data as valid, it takes precedence over hypothetical,
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The threat of solipsism is false; not a real threat at all. It is based on a misunderstanding of the human mind, which does not, and cannot exist in isolation from other human minds. One's own self and mind are learned (acquired) from socialization and cannot ever be separated from that context. The image of Rodin’s solitary thinker is profoundly misleading. We are not monads, and never have been.
The philosophical problem of solipsism is posed by abstracting one’s own mind from that of others, but this abstraction presupposes that the world is already given as a shared world. Hence solipsism presupposes its own refutation. It is a confusion, not a valid proposition.
True solipsism would require that I do not experience myself as a single self in distinction from other selves, but that I experience myself as the only self that exists. But that is impossible, for self is only defined by other. So again, solipsism is impossible in principle.
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Suppose a philosopher, using reason and analysis, abstracts the personal self away from its social origins and maintenance, and considers it as an absolute, transcendental ego, disconnected from all others. From that position of the abstracted transcendental ego, could solipsism be taken seriously?
Husserl, inventor of the transcendental ego, might seem to have believed that. But he also wrote that only his reflections on intersubjectivity make “full and proper sense” of the transcendental ego (Husserl cited by Zahavi, 1996). This is why Husserl claims that a phenomenological discussion of subjectivity in the end turns out to be a discussion not simply of the I, but of the we. Thus once again, even from the position of the transcendental ego, solipsism is not possible in principle.
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Reference
Zahavi, D. (1996). Husserl's Intersubjective Transformation of Transcendental Philosophy. Journal of the British Society for Phenomenology 27 (3), 228-245.