1. Wittgenstein describes the kind of investigation he is engaged in in the PI as ‘a grammatical one’ (PI §90). He speaks of looking for ‘grammatical difference[s] ‘ (PI, PPF §62) and of ‘the rules of grammar’ (PI §497); and he makes a distinction between the ‘surface grammar’ of a word and its ‘depth grammar’ (PI §664). He traces the misunderstandings which he holds to lie at the root of philosophical problems and paradoxes to ‘grammatical illusions’ (PI §110) and to the creation of ‘grammatical fiction[s]’ (PI §307). It is clear that he places the concept of grammar and grammatical investigation at the heart of his understanding of the aims and methods of his later philosophy. Understanding what he means by the terms ‘grammar’, ‘depth grammar’ and ‘grammatical investigation’ is therefore fundamental to an understanding of PI. ‘What is Wittgenstein's conception of grammar, depth grammar and grammatical investigation?’ is one of the most important questions to ask when approaching the task of understanding his later work.

One issue raised by this question is how Wittgenstein's conception of grammar and grammatical investigation relates to the traditional understanding of the notion of grammar. It is clear that Wittgenstein does not use the term ‘grammar’ in a way which implies a clear separation between syntax (grammar) and semantics (meaning). The conception of grammar associated with the traditional grammatical categories – sentence, noun, noun phrase, adjective, transitive and intransitive verbs, adverb and so on – and their permitted concatenations is one that abstracts from the meaning of words. Wittgenstein, by contrast, ‘want[s] to say the place of a word in grammar is its meaning’ (PG §59). This linking together of grammar and semantics is not, however, unique to Wittgenstein. There is a conception of a ‘philosophical grammar’ which is concerned with the general semantic types that are correlated with the syntactic categories of traditional grammar, and with their potential for combination in significant sentences. The aim of such a grammar is to identify the basic types of logico-semantic elements which comprise our conceptual scheme – for example, individual substance, property of an individual substance, event, property of an event, state of affairs and so on – and which might be held to underpin distinctions in syntactic category.