Plato connects the doctrine of the immortality of the rational soul with the theory of Ideas by means of the doctrines of recollection and transmigration. According to the former doctrine, all knowledge is recollection of what was experienced by the soul in its disembodied state before birth. It must carefully be noted, however, that the word knowledge is here used in the special and restricted sense of Plato. Not everything that we should call knowledge is recollection. The sensuous element in my perception that this paper is white is not recollection, since, as being merely sensuous, it is not, in Plato's opinion, to be called knowledge. Here, as elsewhere, he confines the term {213} to rational knowledge, that is to say, knowledge of the Ideas, though it is doubtful whether he is wholly consistent with himself in the matter, especially in regard to mathematical knowledge. It must also be noted that this doctrine has nothing in common with the Oriental doctrine of the memory of our past lives upon the earth. An example of this is found in the Buddhist Jàtakas, where the Buddha relates from memory many things that happened to him in the body in his previous births. Plato's doctrine is quite different. It refers only to recollection of the experiences of the soul in its disembodied state in the world of Ideas. The reasons assigned by Plato for believing in this doctrine may be reduced to two. Firstly, knowledge of the Ideas cannot be derived from the senses, because the Idea is never pure in its sensuous manifestation, but always mixed. The one beauty, for example, is only found in experience mixed with the ugly. The second reason is more striking. And, if the doctrine of recollection is itself fantastic, this, the chief reason upon which Plato bases it, is interesting and important. He pointed out that mathematical knowledge seems to be innate in the mind. It is neither imparted to us by instruction, nor is it gained from experience. Plato, in fact, came within an ace of discovering what, in modern times, is called the distinction between necessary and contingent knowledge, a distinction which was made by Kant the basis of most far-reaching developments in philosophy. The character of necessity attaches to rational knowledge, but not to sensuous. To explain this distinction, we may take as our example of rational knowledge such a proposition as that two {214} and two make four. This does not mean merely that, as a matter of fact, every two objects and every other two objects, with which we have tried the experiment, make four. It is not merely a fact, it is a necessity. It is not merely that two and two do make four, but that they must make four. It is inconceivable that they should not. We have not got to go and see whether, in each new case, they do so. We know beforehand that they will, because they must. It is quite otherwise with such a proposition as, "gold is yellow." There is no necessity about it. It is merely a fact.