Questions like "what is the meaning of a word?" paralyze us because we feel there must be a thing called "meaning" that we ought to be able to point to. We should ask a different question: "what is an explanation of the meaning of a word?" We should learn the grammar of the word "meaning" from the form these explanations take.
Ostensive definition—by which I explain a word by pointing to the thing it denotes—involves an act of interpretation. If say "pencil" to a non-English speaker while pointing to a pencil, that person might interpret "pencil" to mean "wood" or "one" or "hard." So perhaps it is the act of interpretation that gives a word meaning.
If we define understanding as interpretation, we conceive language as signs for words that are dead until a mental process, understanding, gives them life. There is the mechanical process of manipulating these signs, and the organic process of giving them meaning. But if the mental process of understanding "red" is simply a matter of picturing the color in the mind, we could replace understanding the word with the physical process of looking at a chart with colors in one column and the words for these colors in the other column. So we are wrong to think of understanding words as a mental process, Wittgenstein argues. Words are brought to life not by mental processes, but by their grammatical use.
We are often puzzled by how the mind and mental processes work, not because they are complicated, but because we are confused. We think of the mind as the substance through which mental phenomena pass. We think of the mind this way not because of any experimental evidence, but from a feeling that something of the sort must exist.
If thinking is a matter of operating with signs, then we can think with the hand when writing or with the larynx when speaking. We speak literally when we say thinking takes place on paper or in the mouth, but we speak metaphorically when we say thinking takes place in the head. Thinking with the hand and mouth is not analogous to thinking with the head. If the cases were analogous, someone could locate where in the head a particular thought takes place. We could say, for example, "the visual image is located two inches behind the bridge of my nose." Perhaps someone could make such a claim, but we would not understand it, for we have not learned that way of using those words. Similarly, we do not understand the diviner's claim that he feels in his hand that there is water three feet under the earth. It is not simply that we do not know the diviner's art. We also do not know what it would mean to have a feeling in one's hand that corresponds with an underground current. If we are to understand him, he must explain how he came to learn how to estimate the depth of underground currents.
There are two ways of explaining how we come to learn things, two ways of answering the question "why do you proceed as you do?" The first is an explanation of a cause, in which we hypothesize about how our environment has shaped us. The second explanation is a justification our behavior based on a set of rules or norms. Both identifying causes and following rules answer the question "why do you proceed as you do?", so we think of the two explanations as analogous.
Throughout the Blue Book, and in Wittgenstein's general philosophy, we find an emphasis on grammar. This emphasis is motivated by the conviction that we cannot understand the meaning of words and expressions unless we understand how they are used. The word "meaning" itself is the first example Wittgenstein uses. He says we will get nowhere if we think of meaning as a "thing" whose nature we have to unravel. We must first see how the word itself is used before we can even determine what kind of a "thing" it is.
Grammatical investigation must come before scientific investigation. Scientific investigation takes place when we observe a certain definite thing or process in order to determine precisely how this thing or process works. Grammatical investigation determines what sort of a thing or process we are dealing with. Wittgenstein's example of the diviner is a case in point. A scientific investigation of a diviner's art would investigate how the diviner manages to feel underground currents in his hand. However, first we should understand what it means to feel underground currents in one's hand. A grammatical investigation of the word "feel" will show that we can use the word "feel" to refer to objects that we are touching, or we can use it to refer such things as pains, itches, and tingling. We are mystified by the diviner not simply because we ourselves cannot feel underground currents, but because we have no idea what such a feeling would be like.
This is not to say that the diviner is necessarily a fraud. If the diviner were able to explain that certain tingling sensations in his hand corresponded to water at certain depths, and if he were able to explain how he came to learn this correspondence between tingling sensations and water depth, we could understand him. But that would be because the diviner had explained to us the grammar of "feeling in my hand that there is an underground current." Only when we understand a phrase like that can we begin any kind of fruitful scientific investigation.
Wittgenstein uses this distinction between grammatical and scientific investigation to point out the problem with psychology. Psychology makes itself out to be a scientific investigation directed toward understanding the mind and mental processes, but we have yet to sort out what it is we're talking about when we talk about the mind and mental processes. Wittgenstein is not saying there is no such thing as a mind or mental processes, he is saying that we have not adequately defined what we mean by "mind" and "mental processes." All our talk about the mind is metaphorical. We think of imagining as a mental picture, of thinking as a string of mental words, and so on. There is nothing wrong with this kind of metaphorical talk in itself, but we cannot build a scientific theory on it. We mistake all of this metaphorical talk for literal talk, and begin formulating theories about how words and images are processed by the mind.
Whereas we can literally and precisely point to the location of written and spoken thoughts, we can do no such thing with mental thoughts. We cannot say, "this thought exists two inches behind the bridge of my nose." Or if we do say such a thing, we would be in the diviner's position of saying something that as yet has no sense. When we talk about thoughts as existing "in the head," we are speaking metaphorically, and have not provided the basis for any kind of scientific theory.
If there are analogies between what happens in the head and what happens outside the head, then we should be able to replace one with the other. If I understand "fetch me a red flower" because I have a mental image of the color red, I should be able to understand that order just as well by holding a red piece of paper. The language we use to describe mental processes is analogous to the language we use to describe physical processes, and yet we want to claim that the mind is capable of things we could not do simply by looking at physical objects. Not only have we failed to explain the mysteries of the mind, we have failed to define what it is that distinguishes mental processes from physical processes.