It is perhaps true that one of the most important moral qualities of a man, especially an undergraduate, is a knowledge of where to ‘draw the line’; it is certainly true that one of the most essential parts of a lawyer's equipment is the capacity for drawing distinctions correctly. The whole framework of the law is based upon distinctions, and the drawing of false distinctions is as disastrous as is the failure to draw those that are based upon sound reasoning. It is the object of this article to consider, very tentatively, two distinctions which have been introduced into the common law relating to injury done to others by the property of the defendant: in the first place, the distinction between those things which are dangerous per se and those things which are dangerous sub modo, and in the second place, the distinction between the natural and the non-natural user of land. I shall then endeavour to consider the relation of these two problems to each other. But there will be no attempt to state the nature or extent of the liability that arises; for example, I shall not consider the true nature of the rule in Rylands v. Fletcher or the extent of the duty owed by him who deals with dangerous chattels, though some light may incidentally be thrown upon such matters.