In the introduction of the critique of pure reason, Kant says
"That all our knowledge begins with experience there can be no doubt. For how is it possible that the faculty of cognition should be awakened into exercise otherwise than by means of objects which affect our senses. [...] But, though all our knowledge begins with experience, it by no means follows that all arises out of experience. For, on the contrary, it is quite possible that our empirical knowledge is a compound of that which we receive through impressions, and that which the faculty of cognition supplies from itself"
but then he defines "a priori knowledge" in the following way
"in speaking of knowledge which has its sources in experience, we are wont to say, that this or that may be known a priori, because we do not derive this knowledge immediately from experience, but from a general rule, which, however, we have itself borrowed from experience. [...] By the term “knowledge a priori,” therefore, we shall in the sequel understand, not such as is independent of this or that kind of experience, but such as is absolutely so of all experience."
These two statements seem to contradict one another, because if all knowledge has its deepest source in experience, then there is some degree of "a posteriori" in every synthetic judgement. For example, in physics one can say "energy is conserved", but there is no way that this is a priori, as we draw this conclusion by looking at the world.