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a lay-person receives some money and, stating the circumstances, asks us professionals whether she can keep it?

'No' is the answer, 'you must give it back'

'you mean it's unlawful for me to keep it?'

'it was lawful for you to have received the money, but unjust for you to retain it'

'but is it a rule of law which requires me to give it back?'

'yes'

'do you mind if i tell my husband that i have to give it back because it's against the law for me to keep it?'

'yes, so long as you're careful and don't say it's unlawful'

there are circumstances where there is liability on the legal ground that the recipient did not get title, and there are cases where there is liability on the legal ground of 'unjust enrichment'. in each case it is a rule of law that mandates the result and in each case most of us [and the laity] would characterize the retention as 'unjust'

it would be useful i think for us to eschew the expression 'unjust' except as referring to behaviour so characterized in the substantive law of UE --- certainly avoiding the term as a way of expressing subjective dissatisfaction with a result