Full Idea
In terms of formalized quantification theory, each category comprises the range of some distinctive style of variable.
Gist of Idea
In formal terms, a category is the range of some style of variables
Source
Willard Quine (Existence and Quantification [1966], p.92)
Book Reference
Quine,Willard: 'Ontological Relativity and Other Essays' [Columbia 1969], p.92
A Reaction
I add this for those who dream of formalising everything, but be warned that even Quine thought it of little help in deciding on the categories. Presumably there would be some variable that ranged across tigers.