In derivative knowledge our ultimate premises must have some degree of self-evidence, and so must their connection with the conclusions deduced from them. Take for example a piece of reasoning in geometry. It is not enough that the axioms from which we start should be self-evident: it is necessary also that, at each step in the reasoning, the connection of premise and conclusion should be self-evident. In difficult reasoning, this connection has often only a very small degree of self-evidence; hence errors of reasoning are not improbable where the difficulty is great.