It is perfectly consistent with the views maintained in this Lecture that the courts have been very slow to withdraw questions of negligence from the jury, without distinguishing [127] nicely whether the doubt concerned the facts or the standard to be applied.  Legal, like natural divisions, however clear in their general outline, will be found on exact scrutiny to end in a penumbra or debatable land.  This is the region of the jury, and only cases falling on this doubtful border are likely to be carried far in court.  Still, the tendency of the law must always be to narrow the field of uncertainty.  That is what analogy, as well as the decisions on this very subject, would lead us to expect.

The growth of the law is very apt to take place in this way.  Two widely different cases suggest a general distinction, which is a clear one when stated broadly.  But as new eases cluster around the opposite poles, and begin to approach each other, the distinction becomes more difficult to trace; the determinations are made one way or the other on a very slight preponderance of feeling, rather than of articulate reason; and at last a mathematical line is arrived at by the contact of contrary decisions, which is so far arbitrary that it might equally well have been drawn a little farther to the one side or to the other, but which must have been drawn somewhere in the neighborhood of where it falls. 1

In this way exact distinctions have been worked out upon questions in which the elements to be considered are few.  For instance, what is a reasonable time for presenting negotiable paper, or what is a difference in kind and what a difference only in quality, or the rule against perpetuities.

An example of the approach of decisions towards each other from the opposite poles, and of the function of the jury midway, is to be found in the Massachusetts adjudications, [128] that, if a child of two years and four months is unnecessarily sent unattended across and down a street in a large city, he cannot recover for a negligent injury; 1 that to allow a boy of eight to be abroad alone is not necessarily negligent; 2 and that the effect of permitting a boy of ten to be abroad after dark is for the jury; 3 a coupled with the statement, which may be ventured on without authority, that such a permission to a young man of twenty possessed of common intelligence has no effect whatever.

Take again the law of ancient lights in England.  An obstruction to be actionable must be substantial.  Under ordinary circumstances the erection of a structure a hundred yards off, and one foot above the ground, would not be actionable.  One within a foot of the window, and covering it, would be, without any finding of a jury beyond these facts.  In doubtful cases midway, the question whether the interference was substantial has been left to the jury. 4 But as the elements are few and permanent, an inclination has been shown to lay down a definite rule, that, in ordinary cases, the building complained of must not be higher than the distance of its base from the dominant windows.  And although this attempt to work out an exact line requires much caution, it is entirely philosophical in spirit. 5