You know what someone else is thinking and feeling by observing them. But how do you know what you are thinking and feeling? This is the problem of self-knowledge, and Alex Byrne tries to solve it. The basic idea is that you do not discover what you are thinking and feeling by taking a special non-optical look at your own mind, but instead by an inference from a premise about your (typically non-mental) environment. So, for example, you come to know that you believe that it's raining by an inference from a premise that is not about you or anything mental, namely that it is raining. That might well seem like a very odd-even mad- inference, and the book tries to explain why it is in fact a good one.