Solipsism

philosophy

Definition

Solipsism

Solipsism is the philosophical position that nothing can be known or asserted with certainty beyond one’s own mind. On the strongest reading, only one’s own mind and its contents exist; the external world and other minds are, at best, uncertain.

More precisely, let $S$ be the subject and $M_S$ the contents of $S$‘s perception and thought. Solipsism is the claim that the set of things whose existence $S$ can justify is exactly $\{M_S\}$ — everything outside $M_S$ is either unknowable or nonexistent.

Variants

Solipsism separates into three theses of increasing strength:

Epistemological Solipsism

Definition

Epistemological Solipsism

Epistemological solipsism is the weakest form of solipsism. It holds that the external world and other minds may exist, but that their existence cannot be justified from the contents $M_S$ of one’s own mind alone.

Formally, for a subject $S$ with mental contents $M_S$, epistemological solipsism is the claim

$$\forall x\notin M_S:S\text{ cannot justify }┌x\text{ exists}┐.$$

It is a thesis about the limits of perception and inference, not about what exists.

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Metaphysical Solipsism

Definition

Metaphysical Solipsism

Metaphysical solipsism is the strongest form of solipsism. It holds that only one’s own mind and its contents $M_S$ exist; the external world and other minds do not.

Formally, for a subject $S$ with mental contents $M_S$, metaphysical solipsism is the ontological claim

$$\{x:x\text{ exists}\}=M_S.$$

Everything apparently external — bodies, other subjects, the physical world — is taken to be content within $M_S$ rather than an independent entity.

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Methodological Solipsism

Definition

Methodological Solipsism

Methodological solipsism is a research stance, not a thesis. One reasons as though only the contents $M_S$ of one’s own mind were given, then asks what can be reconstructed from $M_S$ alone.

Formally, it prescribes the methodological rule

$$\text{assume }\{x:x\text{ is given}\}=M_S,\text{then derive what else follows.}$$

It does not assert that nothing else exists — it brackets the question and works from the inside out.

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Status

Solipsism is rarely defended as a conclusion; it functions mainly as the limit case against which theories of perception, consciousness, and intersubjective knowledge are measured. Its force comes not from being plausible but from being hard to refute without already assuming the external world it questions.