Not all these propositions have the same status, and we shall be able to discuss only a few of them, and only very briefly. Consider, for example, the statement about the sum of the angles of a triangle. One might well ask “Don't you know that this statement is true? It may not seem obvious, but any high-school geometry student who has just studied the proof can prove it for you. And once you have the proof before you, you can no longer deny that it holds true for all cases-in other words, you can know it a priori, and you don't have to go through a separate process of measurement or observation for every triangle, the way you do for every crow that comes along to see whether it is black. But the statement is also synthetic: there is nothing in the definition of triangle' about 180. So here we have a statement that is both necessary (a priori) and synthetic. Thus the rationalist is right there is a least one synthetic necessary proposition.”