§ 127 The Theory of Penal Liability
We now proceed to the main subject of our inquiry, namely, the general principles of penal liability. We have to consider the legal theory of punishment, in its application both to the criminal law and to those portions of the civil law in which the idea of punishment is relevant and operative. We have already, in a former chapter, dealt with the purposes of punishment, and we there saw that its end is fourfold, being deterrent, disabling, retributive, and reformative. The first of these purposes, however, is primary and essential, the others being merely secondary. In our present investigation, therefore, we shall confine our attention to punishment as deterrent. The inquiry will fall into three divisions, relating (1) to the conditions, (2) to the incidence, and (3) to the measure of penal liability.
The general conditions of penal liability are indicated with sufficient accuracy in the legal maxim, Actus non facit reum, nisi mens sit rea—The act alone does not amount to guilt; it must be accompanied by a guilty mind. That is to say, there are two conditions to be fulfilled before penal responsibility can rightly be imposed, and we may conveniently distinguish these as the material and the formal conditions of liability. The material condition is the doing of some act by the person to be held liable. A man is to be accounted responsible only for what he himself does, not for what other persons do, or for events independent of human activity altogether. The formal condition, on the other hand, is the mens rea or guilty mind with which the act is done. It is not enough that a man has done some act which on account of its mischievous results the law prohibits; before the law can justly punish the act, an inquiry must be made into the mental attitude of the doer. For although the act may have been materially or objectively wrongful, the mind and will of the doer may have been innocent.
We shall see later that the mens rea or guilty mind includes two, and only two, distinct mental attitudes of the doer towards the deed. These are intention and negligence. Generally speaking, a man is penally responsible only for those wrongful acts which he does either wilfully or negligently. Then and only then is the actus accompanied by the mens rea. Then and then only do the two conditions of liability, the material and the formal, coexist. In this case only is punishment justifiable, for it is in this case alone that it can be effective. Inevitable accident or mistake—the absence both of wrongful intention and of culpable negligence—is in general a sufficient ground of exemption from penal responsibility. Impunitus est, said the Romans, qui sine culpa et dolo malo casu quodam damnum committit.326
We shall consider separately these two conditions of liability, analysing first the conception of an act, and secondly that of mens rea in its two forms of intention and negligence.327