Full Idea
It is not enough for understanding the nature of myself, that I feel myself to be a thinking substance, one would have to form a distinct idea of what distinguishes me from all other possible minds; but of that I have only a confused experience.
Gist of Idea
I can't just know myself to be a substance; I must distinguish myself from others, which is hard
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1686.07.4/14)
Book Reference
Leibniz,Gottfried: 'The Leibniz-Arnauld Correspondence', ed/tr. Mason,HT/Parkinson,GHR [Manchester UP 1967], p.59
A Reaction
Not a criticism I have encountered before. Does he mean that I might be two minds, or might be a multitude of minds? It seems to be Hume's problem, that you are aware of experiences, but not of the substance that unites them.