§ 146 Mistake of Law
It is a principle recognised not only by our own but by other legal systems that ignorance of the law is no excuse for breaking it. Ignorantia juris neminem excusat. The rule is also expressed in the form of a legal presumption that every one knows the law. The rule is absolute, and the presumption irrebuttable. No diligence of inquiry will avail against it; no inevitable ignorance or error will serve for justification. Whenever a man is thus held accountable for breaking a law which he did not know, and which he could not by due care have acquired a knowledge of, the case is one of absolute liability.
The reasons rendered for this somewhat rigorous principle are three in number. In the first place the law is in legal theory definite and knowable; it is the duty of every man to know that part of it which concerns him; therefore innocent and inevitable ignorance of the law is impossible. Men are conclusively presumed to know the law, and are dealt with as if they did know it, because they can and ought to know it.
In the second place, even if invincible ignorance of the law is in fact possible, the evidential difficulties in the way of the judicial recognition of such ignorance are insuperable, and for the sake of any benefit derivable therefrom it is not advisable to weaken the administration of justice by making liability dependent on well-nigh inscrutable conditions touching knowledge or means of knowledge of the law. Who can say of any man whether he knew the law, or whether during the course of his past life he had an opportunity of acquiring a knowledge of it by the exercise of due diligence?
Thirdly and lastly, the law is in most instances derived from and in harmony with the rules of natural justice. It is a public declaration by the state of its intention to maintain by force those principles of right and wrong which have already a secure place in the moral consciousness of men. The common law is in great part nothing more than common honesty and common sense. Therefore although a man may be ignorant that he is breaking the law, he knows very well in most cases that he is breaking the rule of right. If not to his knowledge lawless, he is at least dishonest and unjust. He has little ground of complaint, therefore, if the law refuses to recognise his ignorance as an excuse, and deals with him according to his moral deserts. He who goes about to harm others when he believes that he can do so within the limits of the law, may justly be required by the law to know those limits at his peril. This is not a form of activity that need be encouraged by any scrupulous insistence on the formal conditions of legal responsibility.
It must be admitted, however, that while each of these considerations is valid and weighty, they do not constitute an altogether sufficient basis for so stringent and severe a rule.382 None of them goes the full length of the rule. That the law is knowable throughout by all whom it concerns is an ideal rather than a fact in any system as indefinite and mutable as our own. That it is impossible to distinguish invincible from negligent ignorance of the law is by no means wholly true. It may be doubted whether this inquiry is materially more difficult than many which courts of justice undertake without hesitation. That he who breaks the law of the land disregards at the same time the principles of justice and honesty is in many instances far from the truth. In a complex legal system a man requires other guidance than that of common sense and a good conscience. The fact seems to be that the rule in question, while in general sound, does not in its full extent and uncompromising rigidity admit of any sufficient justification.