One part of Leibniz's law, saying that if what appear to be two or more things have all their properties in common they are identical and so only one thing.
In its widest and weakest form, the properties concerned include relational properties such as spatiotemporal ones and self-identity.
A stronger version limits the properties to non-relational properties (that is, qualities), and would therefore imply that there could not be, for example, two exactly similar ball-bearings.
Even the weaker version faces objections if we envisage two ball-bearings alone in an otherwise empty universe, or in corresponding positions in the two halves of a symmetrical universe: what property would one of them have and the other lack? (To try to distinguish them by their relations to each other would presuppose that we could already distinguish them, and the same holds of the halves of the symmetrical universe.)
Also see: principle of sufficient reason
M J Loux, ed., Universals and Particulars (1970)