Rationalism holds, in contrast to empiricism, that it is reason, not experience, that is most important for our acquisition of knowledge. There are three distinct types of knowledge that the rationalist might put forward as supporting his view and undermining that of the empiricist.
First, the rationalist might argue that we possess at least some innate knowledge. We are not born, as the empiricist John Locke thought, with minds like blanks slates onto which experience writes items of knowledge. Rather, even before we experience the world there are some things that we know. We at least possess some basic instincts; arguably, we also possess some innate concepts, such as a faculty for language.
Second, the rationalist might argue that there are some truths that, though not known innately, can be worked out independent of experience of the world. These might be truths of logic or mathematics, or ethical truths. We can know the law of the excluded middle, answers to sums, and the difference between right and wrong, without having to base that knowledge in experience.
Third, the rationalist might argue that there are some truths that, though grounded in part in experience, cannot be derived from experience alone. Aesthetic truths, and truths about causation, for instance, seem to many to be of this kind. Two people may observe the same object, yet reach contradictory views as to its beauty or ugliness. This shows that aesthetic qualities are not presented to us by our senses, but rather are overlaid onto experience by reason. Similarly, we do not observe causation, we merely see one event followed by another; it is the mind, not the world, that provides us with the idea that the former event causes the latter.