Existence monism targets concrete objects and counts by individual token. It holds that exactly one concrete object token exists (the One). It represents an interesting and historically important form of monism, albeit one which is widely regarded as deeply implausible. Consider any two concrete individuals, such as you and I. The existence monist must either deny that at least one of us exists, or deny that at least one of us is a concrete object, or hold that we are identical. This is hard to swallow. (It is important to distinguish existence monism from priority monism, which does not have this implausible implication.)

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Historically, existence monism may have been defended by Parmenides, Melissus, Spinoza, and Bradley, though in each case the claim is controversial.[9] Among contemporary philosophers, Horgan and Potrč are probably the leading, and perhaps the only, existence monists.[10] Thus Horgan and Potrč (2000: 249; c.f. 2008: 8) advance the following ontological and semantical theses:

  • There really is just one concrete particular, viz., the whole universe (the blobject).
  • The blobject has enormous spatiotemporal structural complexity, and enormous local variability—even though it does not have any genuine parts.
  • Numerous statements employing posits of common sense and science are true, even though nothing in the world answers directly to these posits.
  • Truth, for such statements, is indirect language-world correspondence.

Note that existence monism should not be confused with the formula: ∃xy(x=y). That is the logician's formula for expressing the claim that exactly one entity exists. The existence monist is making a much weaker (and slightly more plausible) claim. She can allow that many abstract entities exist, she can allow that many spatiotemporal points exist (assuming that she does not follow the supersubstantivalist in identifying objects with regions), and she can allow that many property tokens exist (assuming she does not follow the bundle theorist in identifying objects with compresent property tokens), as long as she maintains that only one concrete object token exists.[11]

In order to properly characterize existence monism, one should first introduce a predicate ‘C’ that denotes the property of being a concrete object. (The notion of being a concrete object is natural and useful, so I trust this is clear enough to work with.) Then one can introduce the formula:

Existence monism: ∃x(Cx & ∀y(Cy → x=y))

The corresponding logical formulae for existence pluralism and nihilism then run:

Existence pluralism: ∃xy (Cx & Cy & xy)

Existence nihilism: ~∃xCx

It is not built into the formulation of Existence monism that the one concretum has any particular nature. It might be my nose or your left foot. It might be material (realist) or mental (idealist) or neutral. Idealist and neutral forms of existence monism may or may not identify the One with some sort of divinity. Materialist and neutral forms of existence monism typically identify the One with the whole cosmos (Horgan and Potrč's “blobject”). Using ‘u’ as a dedicated constant for the cosmos, which may be defined mereologically as the sum of all concreta, one thus reaches:

Existence monism (cosmic): ∃!xCx & Cu

This says that there is exactly one concretum, namely the cosmos.