EXPERIENCE AND CONTEMPORARY NEUROSCIENCE
Once upon a time in philosophy, it was thought that one's knowledge of one's own current mental states is infallible. Thoughts, and mental states in general, were thought to be transparent to the thinker, that is, nothing can be in my mind without my knowing that it is there, whereas, say, my body is not transparent to me in the same sense. It was held that the mind is a totally transparent medium. If I believe that I am in pain, for example, then I am in pain, and if I believe that I am not in pain then I am not, and if a mental state occurs then I am aware that this mental state occurs.
However, Arthur Schopenhauer's and Sigmund Freud's work have revolutionized the way we think about the mind. “Mentality” is no longer synonymous with “consciousness”. Freud, for instance, argued that people often experience thoughts and feelings that are so painful that they cannot bear them. According to him, these thoughts and feelings cannot be banished from the mind, but they can be banished from consciousness, hence constituting the unconscious. Independently of the degree of accuracy of Freud's theory of the unconscious, it is now commonplace that memories or repressed desires that are not subject to conscious control often affect conscious thoughts and behaviour. Propositional attitudes such as beliefs and desires need not be conscious. For instance, I have believed since childhood that 2 + 2 = 4, but only occasionally does it impinge on my conscious life.