The prosecution must identify a plausible motive for the defendant to have committed the crime. This sometimes is very easy – a large sum of money accruing to the defendant from the commission of the offence may be enough. The plausible motive in a theft is to gain possession over the thing that was stolen. The plausible motive in an assault is to satisfy a grudge or express anger towards a provocateur. This is often a very simple matter and requires little in the way of argument.
Sometimes it may not be so straightforward, such as if the defendant has lost a family member and there is no evidence of disharmony in the family. But a person who is psychologically unwell, is troubled by long past issues, or who is thoroughly deceived, may act on a delusion and do something seemingly implausible. Some motives that may be imputed to a person of a certain kind may not be applicable to an average member of the public. Sexual fetishes are such an example. In such cases the prosecution may need to establish proof with the help of expert witnesses such as a psychologist.