Particularly in the Blue Book, Wittgenstein identifies analogy as a great source of philosophical confusion. For instance, because "A has a gold tooth" and "A has a toothache" have a similar grammatical form, we are tempted to draw an analogy between the two sentences and talk about toothaches and gold teeth as if they are similar. We might think that the way in which we cannot know that A has a toothache is similar to the way in which we cannot know that A has a gold tooth. We might think that we cannot see A's toothache in exactly the same way that we cannot see A's gold tooth when A's mouth is shut. We are wrong to think of toothaches and gold teeth as similar, however, because they are fundamentally different. Unlike gold teeth, toothaches are not the kind of thing that we can talk about seeing or not seeing.

Wittgenstein is careful to point out that there is nothing wrong with these analogies in themselves. The fault is not with ordinary language but with the philosophical misconceptions we generate from ordinary language. It is fine to talk about having a go ld tooth and having a toothache in the way that we do; however, we be vigilant about recognizing that an analogy in phrasing does not signify an analogy in meaning.