" It has been said that the doctrine of equitable lien 'was introduced for the sole purpose of furnishing a ground for the specific remedies which equity confers, operating upon particular identified property, instead of the general pecuniary recoveries granted by courts of law' (Pomeroy's Equity Jurisprudence, 5th ed. (Symons) (1941), pars.166 and 1234). In Whitbread & Co. Ltd. v. Watt, Vaughan Williams L.J. referred to the purchaser's lien for his deposit as 'a right which may be said to have been invented for the purpose of doing justice. It is a fiction of a kind which is sometimes resorted to at law as well as in equity'. General statements of this type lend some support for the approach that one should seek to identify a comprehensive principle covering the implication of any type of equitable lien. Apart from broad generalizations such as 'the phrase equitable lien may not ... do much more than express the opinion of the court that the facts give a priority to the party said to have it' (Sexton v. Kessler) however, it is difficult, if not impossible, to formulate any satisfactory statement of the necessary or sufficient circumstances for the implication of an equitable lien which is applicable to any relationship at all (e.g. the trustee's lien over trust assets; the solicitor's lien over the proceeds of an action). I do not propose to essay that task here. It is adequate for present purposes that I identify what I consider to be the circumstances which are sufficient for the implication, independently of agreement, of an equitable lien between parties in a contractual relationship. Those circumstances have, to some extent, been indicated in what has been said above. They are: (i) that there be an actual or potential indebtedness on the part of the party who is the owner of the property to the other party arising from a payment or promise of payment either of consideration in relation to the acquisition of the property or of an expense incurred in relation to it (see Middleton v. Magnay; Whitbread & Co. Ltd. v. Watt; Combe v. Lord Swaythling; (ii) that that property (or arguably property including that property: see Pollock, loc.cit.) be specifically identified and appropriated to the performance of the contract (see per Lord Hanworth M.R., In re Wait; and (iii) that the relationship between the actual or potential indebtedness and the identified and appropriated property be such that the owner would be acting unconscientiously or unfairly if he were to dispose of the property (or, if it be appropriate, more than a particular portion thereof) to a stranger without the consent of the other party or without the actual or potential liability having been discharged. It may be that the above circumstances or tests, particularly (i), would be unduly restrictive if propounded as a statement of exclusion. As has been said however, they are formulated as a statement of what is sufficient rather than of what is essential. Whether or not they exist or are satisfied in a particular case should, like most questions involved in the application of equitable doctrines, be determined by reference to the substance of the transaction rather than its form."