  One main question concerning this list of categories is whether we can discern any underlying structure or organising principle. This is related with another important question: just what is a category? The Manual of Reason answers this second question by giving the etymological analysis of the term ‘padārtha’ (category): ‘padārtha’ is the artha or meaning of a pada or word. The claim is that the Vaiśeṣika categories are in some way the metaphysical correlates of linguistic structures. One way to make this claim more precise would be to note the existence of striking similarities between the Vaiśeṣika system and Sanskrit grammar (cf. esp. Faddegon 1918). Another way is to observe a distinctive pattern of argument employed, in which the hypothesis that a certain type of substance, quality, etc. exists is supported on the ground that it explains some feature of our linguistic practice (for example, the argument that space exists as it explains our use of directional terms.)