We must never forget that every rule is shattered whenever any single element of the situation is unknown, and that happens very easily and frequently. Suppose that I did not have full knowledge of the nature of water, and walked on terra firma to the edge of some quiet, calm pool. When now I presume: water has a body, it has a definite density, it has consistency, weight, etc., I will also presume that I may go on walking over its surface just as over the surface of the earth,--and that, simply because I am ignorant of its fluidity and its specific gravity. Liebman[137] summarizes the situation as follows. The causal nexus, the existential and objective relation between lightning and thunder, the firing of powder and the explosion, are altogether different from the logical nexus, i.e. the mere conceptual connection between antecedent and consequent in deduction. This constitutes the well known kernel of Humian skepticism. We must keep in mind clearly that we never can know with certainty whether we are in possession of all the determining factors of a phenomenon, and hence we must adhere to the only unexceptionable rule: _Be careful about making rules that admit of no exceptions_. There is still another objection to discuss, i.e. the mathematical exception to Humian skepticism. It might be held that inasmuch as the science of justice is closely related in many ways to mathematics, it may permit of propositions a priori. Leibnitz already had said, “The mathematicians count with numbers, the lawyers with ideas,--fundamentally both do the same thing.” If the relationship were really so close, general skepticism about phenomenal sciences could not be applied to the legal disciplines. But we nowadays deal not with concepts merely, and in spite of all obstruction, Leibnitz’s time has passed and the realities of our profession, indeed its most important object, the human being itself, constitute an integrating part of our studies. And the question may be still further raised whether mathematics is really so exempt from skepticism. The work of Gauss, Lobatschewski, Bolyai, Lambert, would make the answer negative. Let us, for once, consider what significance mathematical postulates have. When Pythagoras discovered his proposition in such a way that he first drew a right-angled triangle and then built a square on each of the sides, and finally measured the area of each and compared them, he must at first have got the notion that that also might be merely accidental. If he had made the construction 10 or 100 times with various triangles and these had resulted always identically, only then might he have been justified in saying that he had apparently discovered a theorem. But then his process was just as thoroughly experiential as that of a scientist who says that a bird has never yet been observed to give birth to living young, and that hence all birds lay eggs.