Further, it is held that all science can be taught, and that what can be known in the way of science can be learnt. But all teaching starts from something already known, as we have explained in the Analytics; for it proceeds either by induction or by syllogism. Now, it is induction that leads the learner up to universal principles, while syllogism starts from these. There are principles, then, from which syllogism starts, which are not arrived at by syllogism, and which, therefore, must be arrived at by induction