There has, for a long time, been a debate as to whether, in order to establish dishonesty, one has to show that the perpetrator knew he was being dishonest.
To illustrate; a tourist using public transport from a country in which public transport was free gets on a bus. If that person got off the bus without paying, would he be dishonest?
The Supreme Court recently considered this issue in the case of Ivey v Genting Casinos  UKSC67. That was a case which involved a professional gambler who was playing baccarat and on leaving the casino, was refused his £7.7 million winnings. The gambler said that although he accepted he was edge sorting, a technique of identifying minute differences in the design on the back of cards, he did not think that was dishonest. The skill he applied led him to the conclusion that it was him being skilful, rather than cheating. The Court considered that he genuinely believed this, hence him taking the claim.
The Supreme Court considered both civil and criminal cases on the matter. The Court confirmed that the same test applies to both Courts. In a thoughtful and ground-breaking decision, the judges clarified the law and set out a two-stage test.
They found that when assessing an individual’s state of mind at the time, one must ascertain (subjectively) the actual state of the individual’s knowledge or belief as to the facts. What his actual state of mind was is a matter of fact. It does not have to be reasonable and in that sense, the test is subjective. The question is whether his belief was genuinely held.
Once there was a finding as to his actual state of mind as to the knowledge or belief as to the facts, the question of whether the person’s conduct was dishonest is to be determined by applying the (objective) standards of the ordinary decent person.
Although there can be exceptions, such as a mistake as to the law as in the earlier example, the previous test set out in the case of R v Ghosh  EWCA Crim2 was over-ruled. In that case, the person had to be proven to have known what he was doing was dishonest. Hence a genuinely held belief can be dishonest – he was cheating.