Inexperience
Kant says that all our knowledge is founded on experience. But each new small increment of knowledge is not so founded, and our whole knowledge is made up of the accumulation of these small new increments not one of which is founded upon experience. Our knowledge, then, is founded not on experience but on inexperience; for where there is no novelty, that is to say no inexperience, there is no increment in experience. Our knowledge is really founded upon something which we do not know, but it is converted into experience by memory.
It is like species—we do not know the cause of the variations whose accumulation results in species and any explanation which leaves this out of sight ignores the whole difficulty. We want to know the cause of the effect that inexperience produces on us.