Thinking, by all definitions, is a covert activity, witnessed only by the person engaged in it. The experience of thinking has been variously described as talking to oneself, hearing voices speaking, seeing mental pictures, manipulating mental images. By virtue of our ability to think, we are able to consider objects and events that are absent, and can sometimes solve problems without actually trying out alternative courses of action. I want to consider here what role language may play in these thought processes.
First, however, we should consider some of the difficulties that psychologists have had in gathering information about thinking, and, in fact, in thinking about thinking. The empirical investigation of thought processes began, quite naturally, with an examination of people's introspective accounts of their conscious experiences. A person would be asked to think about some problem and then to describe everything that passed through his mind while he was thinking. Psychologists often found consistencies among different people's reports, especially when they had trained people in the technique of introspecting. Hopes for this method, however, proved overly optimistic.
A major difficulty with the introspective method is that it provides no objective way to check the accuracy of one's impressions. People generally can think of many alternative ways to explain various things that happen to them. So it should not be surprising if there are also alternative ways to analyze one's own conscious experiences while thinking. The only data of introspection are people's accounts of their private experiences.