Faced with the multiplicity of apparently-discrete objects, people, sensations — ‘the ten thousand things’ as Daoists have put it — how on Earth can it be maintained that reality is One?
It’s because it can be seen — is seen! — that there is just the One source, and that One is differentiated into the ‘ten thousand things’ by the actions of the senses.
Famously, Plato touched on this in his descriptions of how Idea was transmuted into Individual imperfect objects.
One analogy by which this somewhat difficult notion can be grasped is by considering the way that a prism breaks the one white light beam that enters it into many separate colours.
Which is truth? That there is one white light? Or multiple different-coloured light? Both are truth, they are the same thing, undifferentiated or differentiated aspects of the One whole. Interestingly, in principle (and quite recently, in practice) the multiple colours can be put back together to make the original white beam. And interestingly, and in practice — your own practice — the differentiated objects of perception can be perceived as being ultimately One.
A non-obvious implication and consequence of the nature of the One is that everything perceived in existence is necessarily unique.
Consider another analogy: take a beautiful decorated vase, before and after it has inadvertently been dropped on the floor and smashed into ‘ten thousand things’.
Before it was one. Now it is many. The many still add up to the one, but putting it back together requires clear perception, skill, patience. Every part of the whole is different, by necessity, as there cannot be two parts of the one whole that are identical. Some parts may be very similar, but similar is not identical.
By analogy it can be seen that in existence, every thing that exists is necessarily, unimpeachably unique — for it comes from the differentiation of the One reality. Similarity there is: identicality there is not.