The principles of substantive law which have been established by the courts are believed to have been somewhat obscured by having presented themselves oftenest in the form of rulings upon the sufficiency of evidence. When a judge rules that there is no evidence of negligence, he does something more than is embraced in an ordinary ruling that there is no evidence of a fact. He rules that [121] acts or omissions proved or in question do not constitute a ground of legal liability, and in this way the law is gradually enriching itself from daily life, as it should. Thus, in Crafton v. Metropolitan Railway Co., /1/ the plaintiff slipped on the defendant's stairs and was severely hurt. The cause of his slipping was that the brass nosing of the stairs had been worn smooth by travel over it, and a builder testified that in his opinion the staircase was unsafe by reason of this circumstance and the absence of a hand-rail. There was nothing to contradict this except that great numbers of persons had passed over the stairs and that no accident had happened there, and the plaintiff had a verdict. The court set the verdict aside, and ordered a nonsuit. The ruling was in form that there was no evidence of negligence to go to the jury; but this was obviously equivalent to saying, and did in fact mean, that the railroad company had done all that it was bound to do in maintaining such a staircase as was proved by the plaintiff. A hundred other equally concrete instances will be found in the text-books. On the other hand, if the court should rule that certain acts or omissions coupled with damage were conclusive evidence of negligence unless explained, it would, in substance and in truth, rule that such acts or omissions were a ground of liability, /2/ or prevented a recovery, as the case might be. Thus it is said to be actionable negligence to let a house for a dwelling knowing it to be so infected with small-pox as to be dangerous to health, and concealing the knowledge. /3/ To explain the acts or omissions in such a [122] case would be to prove different conduct from that ruled upon, or to show that they were not, juridically speaking, the cause of the damage complained of. The ruling assumes, for the purposes of the ruling, that the facts in evidence are all the facts. The cases which have raised difficulties needing explanation are those in which the court has ruled that there was prima facie evidence of negligence, or some evidence of negligence to go to the jury.