attempt to define the conditions under which a war could be just. He laid these out in his historic work, Summa Theologica: First, war must occur for a good and just purpose rather than the pursuit of wealth or power. Second, just war must be waged by a properly instituted authority such as the state. Third, peace must be a central motive even in the midst of violence. School of Salamanca Some 200 years later, the School of Salamanca expanded Thomas's understanding of natural law and just war. Given that war is one of the worst evils suffered by mankind, the adherents of the School reasoned that it ought to be resorted to only when it was necessary to prevent an even greater evil. A diplomatic agreement is preferable, even for the more powerful party, before a war is started. Examples of "just war" are: In self-defense, as long as there is a reasonable possibility of success. If failure is a foregone conclusion, then it is just a wasteful spilling of blood. Preventive war against a tyrant who is about to attack. War to punish a guilty enemy. A war is not legitimate or illegitimate simply based on its original motivation: it must comply with a series of additional requirements: The response must be commensurate with the evil; more violence than is strictly necessary would be unjust. Governing authorities declare war, but their decision is not sufficient cause to begin a war. If the people oppose a war, then it is illegitimate. The people have a right to depose a government that is waging, or is about to wage, an unjust war. Once war has begun, there remain moral limits to action. For example, one may not attack innocents or kill hostages. The belligerents must exhaust all options for dialogue and negotiation before undertaking a war; war is legitimate only as a last resort. Under this doctrine, expansionist wars, wars of pillage, and wars for glory are all inherently unjust. Nature of God Thomas believed that the existence of God is self-evident in itself, but not to us. "Therefore I say that this proposition, "God exists", of itself is self-evident, for the predicate is the same as the subject... Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though