Parmenides, Nietzsche, Eastern religions, and others have argued in opposition that the experience of independent objects is an illusion, and all of existence is a unity. This minority view seems always associated with techniques for achieving a psychological experience of the unity (including oneself), as it is not at all self-evident and may even contradict common sense. Part of the difficulty seems to be that along with unity is assumed homogeneity, which simply cannot be (if all that is, is identical, how could even experience have any variation?). It is much easier to imagine a heterogeneous unity, a connected reality that nevertheless maintains gradients of various sorts.
Is there any possible way to distinguish between a heterogeneous unity and a reductionist ontology? Synchronically, probably not: the most accurate available description of reality can always be interpreted in either mode. Diachronically, though, we have (so far) always found that a particular ontological interpretation must eventually be abandoned, whereas its conceptual mapping under a heterogeneous unity can be naturally subsumed under a new paradigmatic order. This proves nothing metaphysical but does suggest a way to avoid disappointment and dogma.
Under a regime of heterogeneous unity, detectable gradients suggest boundaries and conceptual ontologies that we as observers are free to adopt. Because it is natural to do so, we may even perform what I call opportunistic reification - thinking of the abstractions as inherent to reality. Crucially, though, in such a scenario one would remain aware of this as a mere cognitive tool rather than a metaphysical imputation.
The approach also can be applied to the self. We can consider the body - and the mind - with which we are familiar to be continuous with the heterogenous unity. But it is not an arbitrary and vacuous distinction to also refer to the self: there are gradients, and processes, that the abstraction "I" reasonably and naturally isolates. Nevertheless, on this view, reference to the self is conscious and instrumental reification and not metaphysical identification.
It bears repeating that this is only a perspective, not a metaphysical claim.