Existence monism is a philosophical position that states:
∃x(Cx & ∀y(Cy → x = y)),
where C is the property of being a concrete object.

The following are some historical instances and approximations of this position and include the idealistic, neutral, and absolute kinds of monism.

Advaita

Perhaps the oldest recorded form of existence monism is kept by the Advaita school of Hindu thought. Advaita literally means "not duality".

Being; the One (To Hen)

Parmenides, the pre-Socratic philosopher hailing from Elea, arguably expressed an early form of existence monism. In the few poetic fragments that survived, he states [1] (paraphrased):

• The only ways of inquiry that are thinkable; First: that Being is and that it is impossible for it to be not-Being. Second: that Being is not and that it is necessary for it not to be. But the second path is completely unknowable, for neither can you know that which is not nor can you declare it.
• To "be thought" and "to be" are the same thing.
• That "things that are not, are" cannot prevail. But let reason, not habit, judge for you this refutation spoken by me.
• Being, ungenerated, is imperishable. It must be whole, of a single kind, unshaken, and complete. Nor was it ever nor will it be, since it is now, all together.

Melissus, a fellow of the Eleatic school, also argued that Being/One is eternal, limitless, singular, unified, immutable, full, motionless, and incorporeal [2].

Dao

In the DaoDeJing [3], the Dao is described to be the root, source, and balance of all of existence, as well as an attitudinal and ethical ideal.

Spinoza

Leibniz

Wittgenstein

blobjectivism

Blobjectivism is a word coined by Terry Horgan and Matjaž Potrč to denote the ontological and semantical position which affirms that [4]:

• 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.