In the words of Fredrick Pollock, “Our lady the common law is a very wise old lady, though she still has something to learn in telling what she knows”
The origin of the concept of common law can be traced back to England. It refers to the law which derives its presence from the unwritten, judge-made laws. These are opposed to written or codified laws. So common law in its basic sense is that body of law that has derived from judicial decisions known as case laws rather than from statues like The Indian Contract Act, 1872.
In every law, whether it is written or unwritten, there are always some defenses in which a defendant can take an excuse against the wrongful act. Defenses or in layman’s language excuses that help a person in escaping the liability, if his action is qualified under the provisions of defenses. So, defenses are also there in the common law.
Out of the defenses’ categories, one such defense in common law is a Mistake. A mistake is an error in understanding facts, meaning, or words of the law. In common law, Mistakes are of two types- Mistake of law and Mistake of Fact. While the former is not a valid defense to be taken in the court of law, but the latter is a valid one. It can be excused on the ground of criminal acts. The difference between the two can be understood in a detailed way as follows.
MISTAKE OF LAW
Generally, when it comes to the Mistake of Law, a Latin maxim is used to explain the same provision. The Latin maxim ‘Ignorantia Juris non-excusat ‘meaning ignorance of the law is no excuse. The principle is sometimes assumed to be equivalent to ‘Every person is presumed to be aware of the law’.
The court thinks that whoever lives in the country is assumed to know the law of the same country in which he resides. That is why the mistake of law is not considered as a defense in the Indian Penal Code (IPC) as well as in the law of torts.
A has murdered B. In this case, A cannot say that he didn’t know that doing murder was a crime and he wasn’t aware of the fact that it is an offense under criminal law. Here, he is not eligible to take the defense of mistake of law.
In the case, Grant V. Borg, the person was charged under the Immigration Act, 1971. Here the defendant stayed beyond the time limit he was allowed to stay. He cannot take the defense of the law in this case because his act is not legal under any circumstances. 
But this presumption was found to have a fault in it. The honorable Supreme Court of India in Motilal Padampat Mills Ltd. V. State of Uttar Pradesh observed that:
“It must be remembered that there is no presumption that every person knows the law. It is often said that everyone is presumed to know the law, but that is not a correct statement: there is no such maxim known to the law”. 
MISTAKE OF FACT
The Mistake of fact is generally explained by a Latin maxim ‘ignorantia facti doth excusat ignorantia Juris non-excusat’ meaning a person has done an act which by law is an offense, under a misconception of facts leading to him to believe in good faith that he was commanded by the law. This is an exception and can be used to reduce or eliminate the liability of the person. It arises when a person does any act but misunderstood some facts that negate an element of the wrong/crime.
It is essential that the mistake of fact should be honest and reasonable. In other words, it must bona fide be found in nature. Section 79 of the Indian Penal Code says that any person who is under the belief of mistake of fact because he believes himself to be justified by law.
In Chirangi V. the State of M.P.  case the court held that there was a mistake of fact that immunized him from the liability. There the defendant had no intention to kills his son.
So, the defense of mistake of fact can be taken when a person has a good intention, he under a mistaken belief in good faith and believed to be justified by law.
In conclusion, it can be clearly stated that the mistake of law is no defense because it is presumed that every person except lunatics, insane, or minors knows and understands the law. On the other hand, a Mistake of fact can be taken as a defense when that act is done in a bona fide manner. If the mistake of law was given as a defense to the violation of the law, then every person would have taken the defense and the justice wouldn’t have been served. It was very likely that this defense could have been misused by the public. There are various perspectives by which the mistake of law and mistake of fact are seen. These are conservative view, liberal view, moderate view, and equivalence view.
 POLLOCK FREDERICK, THE GENIUS OF THE COMMON LAW, 1911.
 AIR 1982 1 WLR 638 HL
 AIR 1979 118 ITR 326 (SC)
 AIR 1952 53 CRLJ 1212
· IPLEADERS, https://blog.ipleaders.in/common-law-apply-india/ (last visited on Jan 17, 2021)
· IPLEADERS, https://blog.ipleaders.in/mistake-fact-mistake-law-defence/ (last visited on Jan 17, 2021)
· IIFL SECURITIES, https://www.indiainfoline.com/article/news-sector-others/ignorance-of-law-can-it-be-an-excuse-113111404453_1.html (last visited on Jan 17, 2021)