In all these cases it will be found that there has been a voluntary act on the part of the person to be charged. The reason for this requirement was shown in the foregoing Lecture. Unnecessary though it is for the defendant to have intended or foreseen the evil which he has caused, it is necessary that he should have chosen the conduct which led to it. But it has also been shown that a voluntary act is not enough, and that even a co-ordinated series of acts or conduct is often not enough by itself. But the co-ordination of a series of acts shows a further intent than is necessarily manifested by any single act, and sometimes proves with almost equal certainty the knowledge of one or more concomitant circumstances. And there are cases where conduct with only the intent and knowledge thus necessarily implied is sufficient to throw the risk of it on the actor. For instance, when a man does the series of acts called [153] walking, it is assumed for all purposes of responsibility that he knows the earth is under his feet. The conduct per se is indifferent, to be sure. A man may go through the motions of walking without legal peril, if he chooses to practise on a private treadmill; but if he goes through the same motions on the surface of the earth, it cannot be doubted that he knows that the earth is there. With that knowledge, he acts at his peril in certain respects. If he crosses his neighbor's boundary, he is a trespasser. The reasons for this strict rule have been partially discussed in the last Lecture. Possibly there is more of history or of past or present notions of policy its explanation than is there suggested, and at any rate I do not care to justify the rule. But it is intelligible. A man who walks knows that he is moving over the surface of the earth, he knows that he is surrounded by private estates which he has no right to enter, and he knows that his motion, unless properly guided, will carry him into those estates. He is thus warned, and the burden of his conduct is thrown upon himself. But the act of walking does not throw the peril of all possible consequences upon him. He may run a man down in the street, but he is not liable for that unless he does it negligently. Confused as the law is with cross-lights of tradition, and hard as we may find it to arrive at perfectly satisfactory general theory, it does distinguish in a pretty sensible way, according to the nature and degree of the different perils incident to a given situation.