In such cases the law goes on a new principle, different from that governing most substantive crimes. The reason for punishing any act must generally be to prevent some harm which is foreseen as likely to follow that act under the circumstances in which it is done. In most substantive crimes the ground on which that likelihood stands is the common working of natural causes as shown by experience. But when an act is punished the natural effect of which is not harmful under the circumstances, that ground alone will not suffice. The probability does not exist unless there are grounds for expecting that the act done will be followed by other acts in connection with which its effect will be harmful, although not so otherwise. But as in fact no such acts have followed, it cannot, in general, be assumed, from the mere doing of what has been done, that they would have followed if the actor had not been interrupted. They would not have followed it unless the actor had chosen, and the only way generally available to show that he would have chosen to do them is by showing that he intended to do them when he did what he did. The accompanying intent in that case renders the otherwise [68] innocent act harmful, because it raises a probability that it will be followed by such other acts and events as will all together result in harm. The importance of the intent is not to show that the act was wicked, but to show that it was likely to be followed by hurtful consequences. It will be readily seen that there are limits to this kind of liability. The law does not punish every act which is done with the intent to bring about a crime. If a man starts from Boston to Cambridge for the purpose of committing a murder when he gets there, but is stopped by the draw and goes home, he is no more punishable than if he had sat in his chair and resolved to shoot somebody, but on second thoughts had given up the notion. On the other hand, a slave who ran after a white woman, but desisted before he caught her, has been convicted of an attempt to commit rape. /1/ We have seen what amounts to an attempt to burn a haystack; but it was said in the same case, that, if the defendant had gone no further than to buy a box of matches for the purpose, he would not have been liable.