Still, in a more limited way, the same principle applies to knowledge that applies to foresight.  It is enough that such circumstances were actually known as would have led a man of common understanding to infer from them the rest of the group making up the present state of things.  For instance, if a workman on a house-top at mid-day knows that the space below him is a street in a great city, he knows facts from which a man of common understanding would infer that there were people passing below.  He is therefore bound to draw that inference, or, in other words, is chargeable with knowledge of that fact also, whether he draws the inference or not.  If then, he throws down a heavy beam into the street, he does an act [56] which a person of ordinary prudence would foresee is likely to cause death, or grievous bodily harm, and he is dealt with as if he foresaw it, whether he does so in fact or not.  If a death is caused by the act, he is guilty of murder. 1 But if the workman has reasonable cause to believe that the space below is a private yard from which every one is excluded, and which is used as a rubbish heap, his act is not blameworthy, and the homicide is a mere misadventure.

To make an act which causes death murder, then, the actor ought, on principle, to know, or have notice of the facts which make the act dangerous.  There are certain exceptions to this principle which will be stated presently, but they have less application to murder than to some smaller statutory crimes.  The general rule prevails for the most part in murder.

But furthermore, on the same principle, the danger which in fact exists under the known circumstances ought to be of a class which a man of reasonable prudence could foresee.  Ignorance of a fact and inability to foresee a consequence have the same effect on blameworthiness.  If a consequence cannot be foreseen, it cannot be avoided.  But there is this practical difference, that whereas, in most cases, the question of knowledge is a question of the actual condition of the defendant’s consciousness, the question of what he might have foreseen is determined by the standard of the prudent man, that is, by general experience.  For it is to be remembered that the object of the law is to prevent human life being endangered or taken; and that, although it so far considers blameworthiness in punishing as not to hold a man responsible for consequences which [57] no one, or only some exceptional specialist, could have foreseen, still the reason for this limitation is simply to make a rule which is not too hard for the average member of the community.  As the purpose is to compel men to abstain from dangerous conduct, and not merely to restrain them from evil inclinations, the law requires them at their peril to know the teachings of common experience, just as it requires them to know the law.  Subject to these explanations, it may be said that the test of murder is the degree of danger to life attending the act under the known circumstances of the case. 1