of a collaboration contract with a practitioner were not in employment; there was thus no question of compulsory insurance for these more or less independent collaborators within the framework of the said employees insurance scheme. 2.3 This situation changed on 14 April 1983, when the Central Appeals Board ruled, contrary to what BVG had previously accepted, that part-time physiotherapists working on an invoicing basis were in fact working in such a dependent socio-economic position vis-à-vis the owner of owners of the practice that their work status was socially comparable to employment and had, therefore, to be regarded as such in the framework of social security insurance legislation. On the basis of this judgement, BVG informed the national professional organizations of physiotherapists that part-time physiotherapists working on an invoicing basis henceforth would have to be insured and that contributions due would have to be paid by the owner of a physiotherapy practice as if he were an employer. In its circular, the BVG announced that contributions due would be collected from 1 January 1984, on the understanding that those required to pay the contributions would send their names to BVG before 1 January 1985. The collection of contributions for the years prior to 1984 would then be waived. 2.4 Despite the BVG view that, from 1984 onwards, there was no longer any question of such a special situation in respect of the obligation for owners of physiotherapy practices to pay contributions, the authors maintain that physiotherapists are still treated differently with regard to the date of commencement of the obligation to contribute. Thus, it has become apparent that those physiotherapy practices which, at an earlier stage, were unambiguously informed in writing by the association that there was no obligation to contribute, were regarded as liable to pay the first contribution in 1986, whereas practices that had not received a letter sent directly by BVG, in which they were informed that there was no such obligation, were required to pay contributions retroactively to January 1984. 2.5 As soon as the complainants learned that, in the former case, the requirement to pay their contributions could have begun in 1986 and thus did not have retroactive effect to 1 January 1984, they invoked the principle of equality before the law, by means of the appeals procedure then prevailing in the Central Appeals Board. They argued that the situation in their practice had not been essentially different from that in other practices which had learned directly from BVG that no insurance obligation was required with regard to their part-time physiotherapists. The part-time physiotherapist who collaborated the authors was also working on an invoicing basis, as others who collaborated with practices that, before 1983, had learned directly from BVG that there would be no question of an insurance obligation. 2.6 Despite the invocation of the principle of equality before the law, the Central Appeals Board held, in its final judgement in the case on 19 August 1987, that the decision by BVG to demand contributions from the complainants with retroactive effect to 1984 was based on legal rules of a compulsory nature which could not or must not be tested against general principles of law. 2.7 To the authors, the Central Appeals Board thereby implicitly concluded that the acknowledged difference in treatment in the manner of demands for contributions between various physiotherapy practices is in accordance with law. The authors point to what they consider an inconsistency in the Central Appeals Board s judgement. On the one hand, the Board appears to take the view that the application of compulsory legal rules cannot or must not be tested against general principles of law; on the other hand, it appears from established case-law that such rules must not be applied if they are in conflict with the principle of confidence in the law, i.e. the principle of the certainty of the law. The authors question why owners of physiotherapy practices who were not directly informed by BVG in the past that part-time physiotherapists co-operating with them were not subject to social security contributions should be subjected to different and less favourable treatment with respect to contributions due after 1984 than those practitioners who had received such direct information. 2.8 The authors claim that since the principle of confidence in the law can, under certain circumstances, prevent the application of compulsory legal rules, it is all the more surprising that this does not apply to the principle of equality before the law, enshrined in article 1 of the Netherlands Constitution and article 26 of the Covenant. They 64