 The Manual of Reason’s definition of a word is celebrated and often quoted. It says (TS 66) that a word is that which has semantic power (śaktaṃ padam). A sentence (vākya) is defined as a collection of words. So The Manual of Reason’s definition fits with the modern one, according to which a word is a semantically significant sentential constituent. Semantic power is that knowledge of which enables the competent hearer to interpret the word, and this, for the Naiyāyikas, consists in the word’s standing for an object. Thus, someone who understands the word “cat’ knows something of the form &lt;“cat” means cat&gt;, where “means” is a relation between words and objects of some type. The Manual of Reason raises three questions about this relation: (i) Is it a new and irreducible type or relation, not reducible, that is, to any of the relations already admitted (TSD 66a)? (ii) What is the relatum, i.e. with what type of entity are words semantically related (TSD 66b, k)? (iii) How do we learn the meaning of a word (TSD 66c, d).