We have so far dealt with the more theoretical part of theology and religion, so much of it as may be and is accepted by nations and religions other than Jews. It remains now to approach the more practical and the more specifically Jewish phases of religion; though in the purely ethical discussions and those relating to Providence we have once more a subject of general application, and not exclusively Jewish. As the introduction to this second part of the subject, Abraham Ibn Daud devotes a few words to the theoretical defence of tradition, or rather of mediate knowledge. He does so by analyzing the various kinds of knowledge. Knowledge, he says, is either intelligible or sensible. Sensible knowledge is either directly perceived by the subject or received by him from another who perceived it directly, and whom he believes or not as the case may be. That is why some things believed by some people are not believed by others. The ignorant may think that this weakness is inherent in matters received from others. As a matter of fact such indirect knowledge is at the basis of civilization and makes it possible. If every man were to judge only by what he sees with his own eyes, society could never get along; there would be no way of obtaining justice in court, for the judge would not put credence in witnesses, and the parties would have to fight out their differences, which would lead to bloodshed and the disruption of social life. The different attitude of different persons to a given matter of belief is due not necessarily to the uncertainty of the thing itself, but to the manner in which the object of the belief came down to us. If a thing rests upon the testimony of one man, its warrant is not very strong. But if a whole nation witnessed an event, it is no longer doubtful, unless we suppose that the account itself is due to one writer, and the event never happened. We shall discuss these matters in the sequel.[241] Having justified in a general way the knowledge derived from the testimony of others by showing that society could not exist without depending upon such knowledge; though admitting at the same time that caution should be exercised and criticism in determining what traditional testimony is valid or not, we now take up one of these traditional phenomena which plays perhaps the most important rôle in Jewish theology, namely, the phenomenon of prophecy. Before discussing the traditional aspect of this institution and its purpose in the history of religion we must consider it from its natural and psychological aspect.