So far, we have been concerned only with the syntax of English. In this section we shall disscuss how to extract the meanings of sentences. First, we have to know what kinds of things `meanings' are. There is no easy and uncontroversial answer to this, and indeed it is a question that has vexed linguists and psychologists for a century and philosophers for millennia. For a broad review of current semantic theories you might like to look at Language, Meaning and Context by John Lyons, or the denser and more detailed Semantic Theory by Ruth Kempson. In this book, we shall restrict our discussion to only one approach, called formal semantics, which is sometimes also called compositional semantics because of the crucial part played by the principle of compositionality: that the meaning of the whole sentence is composed from meanings of its parts.