What is the coincidence of actus reus and mens rea?

Coincidence of actus reus and mens rea is the principle that both the mental and physical element of the crime must be present in order for someone to be liable for murder. This is often called the contemporaneity rule and is often trickier than it seems.

In some cases it can be difficult to tell when intent came into play and as a result it is difficult for the courts to know if they coincided. The mens rea or actus reus don't have to start at the same time, for example I can have the intent to murder someone before I go out and do it, or I could push someone over by accident and then after realising they are vulnerable/unconscious etc take a blow which kills them, but it must coincide at some point during the process.

If, for example, I was really mad at my neighbour and wanted to do something, like say hurt them, I have the intent to do so. If when I go over there we have a chat and it goes well, I may stop wanting to do that and therefore the intent goes. However the next day when I'm reversing my car I accidentally ride over their foot - this would be the actus reus present of me hurting them, but the mens rea is not present at that time.

There are various cases where the courts had to consider this:

Thabo Meli v R - This is quite a case and is excellent for demonstrating this principle. 4 people had planned to kill a man and make it look like an accident. (guilty mind) They took him, beat him over the head, and believing [wrongly] that he was dead, threw him over a cliff. Medical evidence confirmed that the man had died AFTER being thrown down the cliff due to exposure. They appealed their murder conviction by stating that when the mens rea had developed, he had not died (despite the hit over the head), and that when he did die, the intent was not present (they didn't mean for him to die of exposure) therefore the two did not coincide. Thankfully the judge decided that to separate the two was impossible and that one was a direct result from the other and therefore it would be an artificial separation, but the principle can be seen in action, in the contect of a murder trial.

Fagan v Metropolitan Police - This is case where a man who was arguing with the police from inside his car accidentally by reversing the car ran over an officer's foot. Here the actus reus was present, but due to him being unaware of what was happening, he would not in fact be liable as he did not have the guilty mind (mens rea). He had stopped his car on one of the officers feet - when they alerted him to this, he decided to not move his car - this is when the mens rea developed, because now he knew of the situation, he developed the guilty mind by staying on the officer's foot.