IGNORANCE (Lat. ignorantia, from ignorare, not to know), want of knowledge, a state of mind which in law has important consequences. A well-known legal maxim runs: ignorantia juris non excusat (“ignorance of the law does not excuse”). With this is sometimes coupled another maxim: ignorantia facti excusat (“ignorance of the fact excuses”). That every one who has capacity to understand the law is presumed to know it is a very necessary principle, for otherwise the courts would be continually occupied in endeavouring to solve problems which by their very impracticability would render the administration of justice next to impossible. It would be necessary for the court to engage in endless inquiries as to the true inwardness of a man’s mind, whether his state of ignorance existed at the time of the commission of the offence, whether such a condition of mind was inevitable or brought about merely by indifference on his part. Therefore, in English, as in Roman law, ignorance of the law is no ground for avoiding the consequences of an act. So far as regards criminal offences, the maxim as to ignorantia juris admits of no exception, even in the case of a foreigner temporarily in England, who is likely to be ignorant of English law. In Roman law the harshness of the rule was mitigated in the case of women, soldiers and persons under the age of twenty-five, unless they had good legal advice within reach (Dig. xxii. 6. 9). Ignorance of a matter of fact may in general be alleged in avoidance of the consequences of acts and agreements, but such ignorance cannot be pleaded where it is the duty of a person to know, or where, having the means of knowledge at his disposal, he wilfully or negligently fails to avail himself of it (see Contract).
In logic, ignorance is that state of mind which for want of evidence is equally unable to affirm or deny one thing or another. Doubt, on the other hand, can neither affirm nor deny because the evidence seems equally strong for both. For Ignoratio Elenchi (ignorance of the refutation) see Fallacy.