This is the formula for establishing criminal liability in the United Kingdom.
AR is Actus Reus (Latin for “Guilty Act”), which means that the defendant (i.e. the person accused of a particular crime) has physically committed the actions required to break the law.
MR is Mens Rea (Latin for “Guilty Mind”). This is an interesting point, because most people unfamiliar with the law tend to think that this is not required for a crime to be committed. Surely just the physical act is enough to be guilty of a crime?
Well, no. The maxim “The maxim actus non fit reus nisi mens” (Latin for “An act does not make one guilty unless there is a criminal intent.”).
Why would that be the case? Why not just deal with the physical reality and ignore the messy grey area of “intent”?
In Sweet v. Parsley, the House of Lords gave this summary:
There has for centuries been a presumption that Parliament did not intend to make criminals of persons who were in no way blameworthy in what they did. This means that, whenever a section is silent as to mens rea there is a presumption that, in order to give effect to the will of Parliament, we must read in words appropriate to require mens rea.
As a rather banal example, you cannot commit the criminal act of theft if you did not intend to steal something. If we just limit ourselves to the physical act of taking something that doesn’t belong to us, this can make a criminal of us all. For instance, what if I lend you something? Clearly, that is not yours to own, but there is the presumption that you are able to take it for some time.
In fact, certain actions of taking things that do not belong to you at that moment are positive. The shopkeeper wants you to pick up an apple in his store. It is only then a crime if you knowingly walk out of the shop without paying.
D stands for Defense. The minus sign indicates that there is no proper or acceptable defense for the crime.
L stands for liability, specifically criminal liability.
So if there is Actus Reus and Mens Rea, and there is no Defense, then the defendant is criminally Liable.