Many of the 'knowledge' and 'opportunity' cases, including Boardman v Phipps [1966] UKHL 2; [1967] 2 AC 46, which is a leading case, would seem to require that the benefit to be accounted for be acquired by the use of both the knowledge or the opportunity or of one or other of them. That is the way in which it is put by Lord Hodson in the extract from his speech which I have already reproduced ([1967] 2 AC 105). It is an idea which runs throughout the cases. The liability to account is said to attach to 'those, who by use of a fiduciary position make a profit'; Lord Russell of Killowen in Regal (Hastings) Ltd v Gulliver [1967] 2 AC 134, 144. The liability is said to be to 'account for profits acquired by him by reason of his fiduciary position or by reason of the opportunity and the knowledge, or either, resulting from it': Lord Wright in the Regal case, supra, at 154. Or, as Lord Porter put it in that case (at 158): 'The legal proposition may, I think, be broadly stated by saying that one occupying a position of trust must not make a profit which he can acquire only by use of his fiduciary position, or, if he does, he must account for the profit so made'. And many more statements to the same effect could be extracted from the cases.
The question, however, remains, it being whether it is necessary for a plaintiff claiming an account to prove, that is to say to establish on the balance of probabilities, that the fiduciary did in fact use the knowledge, or the information, or the opportunity which came to him while he stood in that relationship and that he made the profit by such use. In my opinion it is not.
It is enough, I think, to show that the fiduciary gains his knowledge or opportunity within the fiduciary relationship; that it was knowledge or an opportunity which could have been used in the sense that it was relevant to the acquisition of the profit or benefit in fact acquired and that by doing what he did to acquire the profit or benefit the fiduciary placed himself in a position where his duty and interest did conflict or in a position where as a real and sensible possibility they may conflict. If that can be shown, and on the facts of this case the conflict was actual and not merely potential, it is not necessary to go further so as to prove that either the information or the opportunity was in fact used so as to acquire that benefit. The reason for that, I think, rests in policy and it is based, inter alia, on the impossibility of proof. That appears in one of the early cases. In Ex Parte Lacey [1802] EngR 75; (1802) 6 Ves Jun 625 at 627 Lord Eldon is reported to have said: 'I say, whether he makes advantage or not, if the connection does not satisfactorily appear to have been dissolved, it is the choice of the cestui que trusts, whether they will take back the property, or not; if the trustee has made no advantage. It is founded upon this; that though you may say in a particular case that he has not made advantage, it is utterly impossible to examine upon satisfactory evidence and the power of the court, by which I mean, in the power of the parties, in 99 cases out of 100 whether he made advantage or not'. Or, as Lord Eldon expressed it in a later case, Ex Parte James [1803] EngR 536; (1803) 8 Ves 337, 345: 'No court is equal to the examination and ascertainment of the truth in much the greater number of cases'. It is for this reason that the rule that a person in a fiduciary position is not to allow his interest to conflict with his duty is said to be absolute and inflexible. Should he do so then: 'The consequences of such a conflict are not discoverable. Both justice and policy are against their investigation': see Rich, Dixon & Evatt JJ in Furs Ltd v Tomkies [1936] HCA 3; (1936) 54 CLR 583 at 592. And for the same reason, in my opinion, whether a person so placed has in fact used advantages so acquired - knowledge or opportunity - is likewise 'non-discoverable' and proof that he did so is not required. The law was recently so laid down by Gibbs J, as he then was, in Consul Development Pty Ltd v DPC Estates Pty Ltd [1975] HCA 8; (1975) 132 CLR 373 at 393. His Honour there said: 'However, the rule that a person in a fiduciary position is not entitled to make a profit without the knowledge and assent of the person to whom the fiduciary duty is owed is not limited to cases where the profit arises from the use of the fiduciary position or the opportunity or knowledge gained from it. The basis of the rule is that a person in a fiduciary position may not place himself in a situation where his duty and interest conflict. In Aberdeen Railway Co v Blaikie Brothers (1854) 1 Macq 461 at 471; Lord Cranworth LC stated the principle so far as it applies to directors in the following words: 'A corporate body can act only by agents, and it is of course the duty of those agents so to act as best to promote the interests of the corporation whose affairs they are conducting. Such agents have duties to discharge of a fiduciary nature towards their principal. And it is a rule of universal application, that no-one, having such duties to discharge, shall be allowed to enter into engagements in which he has, or can have, a personal interest conflicting, or which possibly may conflict, with the interests of those whom he is bound to protect.'