I realize that categories of physical objects are essentially a human construct. However, even so, it seems to me like everything that exists is part of a larger group of things that are similar in structure and origin. These similarities exist without the necessity of humans to perceive them.

For example, there is not a single atom but many, there is not a single star but many, there is not a single planet but many, there is not a single human but many, etc.

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    I feel you answer your own question by placing everything that exists in the category 'physical objects'. But everything is the only member of some category. For instance, this is an animal in the general category 'cat', but it is the only member of the category of cats that are sitting on my mat.
    – user20253
    Dec 5 '17 at 13:09
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    I? The universe?
    – Conifold
    Dec 5 '17 at 22:31
  • @Conifold I would say that "you" belong to the category of "Humans". The Universe would be a candidate but it seems like cheating to me since I'm asking about objects within the universe; also we don't really know if that's the case since don't really understand its true nature.
    – driangle
    Dec 6 '17 at 1:33
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    If you want it not to fall under any category then asking about "objects" or "things" means that the answer is trivially no, since it will fall under the category of "object" or "thing". And if you only want some category of which it is a unique representative then I, or anything else, works, as PeterJ explained. I do not see how to interpret the question non-trivially.
    – Conifold
    Dec 6 '17 at 21:22
  • Whether there does exist such an object in the physical world might have a purely contingent answer : yes or no. Isn't the more interesting question whether there exists anything, or could exist anything, in the physical world which is logically unique in the sense that it belongs to a category that logically can or could have only one member ?
    – Geoffrey Thomas
    Dec 30 '17 at 16:15

Ιn the philosophy of quantum mechanics some authors who are wave function realists (i.e. who believe that quantum states have an ontology which is independent from our knowledge about them) claim that the "universal wave function" belongs to a unique ontological category which is nothing like anything else we experience.

They make this claim in response to other views about the ontology of the wave function as a field, a law, a blob etc. since all these views face important difficulties.

So I guess there isn't a yes or no answer to your question.It depends on what kind of metaphysics one adopts. I guess a better question would be: do we have reasons to believe that there are physical entities of a unique ontology?

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