+To develop this analogy just a bit further, syntactic categories
+determine which expressions can combine with which other expressions.
+If a word is a member of the category of prepositions, it had better
+not try to combine (merge) with an expression in the category of, say,
+an auxilliary verb, since *under has* is not a well-formed constituent
+in English. Likewise, types in formal languages will determine which
+expressions can be sensibly combined.
+
+Now, of course it is common linguistic practice to supply an analysis
+of natural language both with syntactic categories and with semantic
+types. And there is a large degree of overlap between these type
+systems. However, there are mismatches in both directions: there are
+syntactic distinctions that do not correspond to any salient semantic
+difference (why can't adjectives behave syntactically like verb
+phrases, since they both denote properties with (extensional) type
+`<e,t>`?); and in some analyses there are semantic differences that do
+not correspond to any salient syntactic distinctions (as in any
+analysis that involves silent type-shifters, such as Herman Hendriks'
+theory of quantifier scope, in which expressions change their semantic
+type without any effect on the syntactic expressions they can combine
+with syntactically). We will consider again the relationship between
+syntactic types and semantic types later in the course.
+