From the intro:
Introspection, as the term is used in contemporary philosophy of mind, is a means of learning about one's own currently ongoing, or perhaps very recently past, mental states or processes. You can, of course, learn about your own mind in the same way you learn about others' minds—by reading psychology texts, by observing facial expressions (in a mirror), by examining readouts of brain activity, by noting patterns of past behavior—but it's generally thought that you can also learn about your mind introspectively, in a way that no one else can. But what exactly is introspection? No simple characterization is widely accepted. Although introspection must be a process that yields knowledge only of one's own current mental states, more than one type of process fits this characterization.
Introspection is a key concept in epistemology, since introspective knowledge is often thought to be particularly secure, maybe even immune to skeptical doubt. Introspective knowledge is also often held to be more immediate or direct than sensory knowledge. Both of these putative features of introspection have been cited in support of the idea that introspective knowledge can serve as a ground or foundation for other sorts of knowledge.
Introspection is also central to philosophy of mind, both as a process worth study in its own right and as a court of appeal for other claims about the mind. Philosophers of mind offer a variety of theories of the nature of introspection; and philosophical claims about consciousness, emotion, free will, personal identity, thought, belief, imagery, perception, and other mental phenomena are often thought to have introspective consequences or to be susceptible to introspective verification. For similar reasons, empirical psychologists too have discussed the accuracy of introspective judgments and the role of introspection in the science of the mind.