Anything is only itself because it’s functionally different from everything else. This computer I’m typing at is only itself because it’s neither at the next desk, nor does it function in the manner of an apple, amongst many other things.
The degree of differentiation is not absolute. There is a gradient of order from the inside core to the outside, where others lie. There are, nonetheless, boundaries. Permeable boundaries, but boundaries nonetheless. Discontinuities where the gradient takes a leap.