This becomes especially our duty where, believing ourselves to have discovered some generally valid rule, we are compelled to draw conclusions without the assistance of experts. How often have we depended upon our understanding and our “correct” a priori method of inference, where that was only experience,--and such poor experience! We lawyers have not yet brought our science so far as to be able to make use of the experience of our comrades with material they have reviewed and defined in writing. We have bothered a great deal about the exposition of some legal difficulty, the definition of some judicial concept, but we have received little instruction or tradition concerning mankind and its passions. Hence, each one has to depend on his own experience, and that is supposed to be considerable if it has a score of years to its back, and is somewhat supplemented by the experience, of others. In this regard there are no indubitable rules; everybody must tell himself, “I have perhaps never experienced this fact, but it may be that a thousand other people have seen it, and seen it in a thousand different ways. How then, and whence, my right to exclude every exception?”
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.