The distinction tentatively drawn by Mr. Porteous at the last meeting of the Society between logical and causal necessity depends on the more general distinction between what is known or capable of being known as self-evident and what is known only as matter of fact. That there are three cows in a field is a matter of fact. That 1 + 2 = 3 is self-evident and necessarily true (if it is true at all). So soon as the question is raised it is seen that there can be no alternative to its being true—otherwise it is not self-evident. But there is no necessity that a field should have cows in it, or that if it has they should be just three in number. Similarly, it does not seem self-evident that an unsupported stone must fall to the ground.