In his Critique of Pure Reason, Immanuel Kant is concerned with the nature of human knowledge, or epistemology. He is often described as having revolutionized philosophy by showing how the subjective and objective are linked together through the synthetic a priori concepts to create human knowledge.
The self in Kant is both the knowing subject and the object of knowledge, both the "I" that I attach to all my representations and itself a representation to which I attach the "I". The "I" is both perceiver and perceived.
The self as perceiver cannot be known, but because the self is also perceived, and thus the object of apperception, it can be known as an object (an "object of apperception") filtered through the same categories and synthetic a priori intuitions as other possible objects of knowledge.