This is a ‘metalinguistic’ usage, which uses words not only to express their ordinary meanings but also to subject their meanings to critical analysis.
It is drawn from an essay ‘On Nothing’, which (with some degree of playfulness) inquires into the meaning of the word nothing—particularly as it contrasts with the notion of something.
This requires the author to distinguish between
- the word something,
- particular entities which the word something may designate, and
- the set of entities to which the word something may refer.
In your sentence the author is referring to #3: a something is some particular member of the set ‘something’.
This is an unusual usage, because ordinary language is not designed for talking about itself; but under the circumstances it is entirely proper.