Now Plato did not clearly make this distinction between necessary and non-necessary knowledge. But what he did perceive was that mathematical knowledge can be known without either experience or instruction. Kant afterwards gave a less fantastic explanation of these facts. But Plato concluded that such knowledge must be already present in the mind at birth. It must be recollected from a previous existence. It might be answered that, though this kind of knowledge is not gained from the experience of the senses, it may be gained from teaching. It may be imparted by another mind. We have to teach children mathematics, which we should not have to do if it were already in their minds. But Plato's answer is that when the teacher explains a geometrical theorem to the child, directly the child understands what is meant, he assents. He sees it for himself. But if the teacher explains that Lisbon is on the Tagus, the child cannot see that this is true for himself. He must either believe the word {216} of the teacher, or he must go and see. In this case, therefore, the knowledge is really imparted from one mind to another. The teacher transfers to the child knowledge which the child does not possess. But the mathematical theorem is already present in the child's mind, and the process of teaching merely consists in making him see what he already potentially knows. He has only to look into his own mind to find it. This is what we mean by saying that the child sees it for himself. In the "Meno" Plato attempts to give an experimental proof of the doctrine of recollection. Socrates is represented as talking to a slave-boy, who admittedly has no education in mathematics, and barely knows what a square is. By dint of skilful questioning Socrates elicits from the boy's mind a theorem about the properties of the square. The point of the argument is that Socrates tells him nothing at all. He imparts no information. He only asks questions. The boy's knowledge of the theorem, therefore, is not due to the teaching of Socrates, nor is it due to experience. It can only be recollection. But if knowledge is recollection, it may be asked, why is it that we do not remember at once? Why is the tedious process of education in mathematics necessary? Because the soul, descending from the world of Ideas into the body, has its knowledge dulled and almost blotted out by its immersion in the sensuous. It has forgotten, or it has only the dimmest and faintest recollection. It has to be reminded, and it takes a great effort to bring the half-lost ideas back to the mind. This process of being reminded is education.