In mathematics, indexes are useful to discriminate elements of a set. These elements have common characteristics, symbolized by a same name. The indexes make it possible to differentiate each one of these elements.
In general semantics, we use indexes with a similar aim. Our goal is to individualize the elements of a set, so as to point out their differences to us, differences that the aristotelian law of identity very often makes us forget. The use of indexes enables us to take into account the first premise ("is not"), at least at the descriptive level (first verbal level).
We can thus say "Chair1 is not chair2", "Smith1 is not Smith2", "TimeSmith is not timeBrown", etc.
Whenever you use indexes, try to find some of the differences between the objects considered.