Wittgenstein writes that there seem to be two ways of explaining the meaning of a word: through 'verbal' and through 'ostensive' definitions. A verbal definition explains a statement with the help of another statement. An ostensive definition is, in the words of McGinn, 'an act of giving the meaning of a wordby pointing to an exemplar'. Wittgenstein adds: 'The verbal definition, as it takes us from one verbal expression to another, in a sense gets us no further. In the ostensive definition however we seem to make a much more real step towards learning the meaning.' Obviously, verbal definitions only make connections within language, and do not bring us from language to reality. Therefore, ostensive definitions appear to be the only way of connecting words with something outside language. Baker and Hacker offer the following summary of the enormous importance of ostensive definitions, at least for the view of language criticized by Wittgenstein:
ostensive definitions provide the only possible means for correlating words with things. Only an utterance of the form 'That is ...', together with the gesture of pointing at something, can be used to correlate a word with a thing. There must be ostensive definitions in every language. They are necessary for language to represent reality. [...] Every ostensive definition forges a link between language and the world.
Through ostensive definition 'we seem to pass beyond the limits of language and to establish a connection with reality itself', writes Wittgenstein. But, to that end, the connection established by the ostensive definition must be unequivocal, infallible and definitive. 'Otherwise, ostensive definition could not provide the foundations of language. If every ostensive definition were ambiguous or left open questions about the application of the defined word, it would require supplementation', conclude Baker and Hacker, '[a]ny attempt to supplement an ostensive definition [...] must be either redundant or inconsistent with the meaning already assigned to it'.
quoting Baker and Hacker's Understanding and Meaning, p36
den Dulk later goes into the problem of ostensive definitions, using a desk as an example: when you point to a desk and say that desk is "rectangular", "furniture", "brown" etc you don't know which of the labels means which aspect of the desk. in fact, this act of definition actually takes place within language, with my pointing finger and the desk itself being part of the grammatical structures of language, despite not being words (though they can be replaced with words, which is indicative). in other words, "an ostensive definition supplies a linguistic rule, not a justification of that rule", which means that you can't justify grammar by referring to reality!