The approach of the natural lawyers
Nevertheless, by accepting these exceptions, Donellus was still moving within the framework set out by the Roman sources. It was left to the natural lawyers to challenge traditional doctrine in a more fundamental way.[3488]’8 Discarding the subtleties of Roman law, they found an altogether new starting point for determining the effect of initial impossibility on contractual obligations in the idea that (in the words of Grotius:) "de verbintenisse is een gebruick van eens mensche vrije macht".'09 The content of a contractual obligation is attributable to the promisor only if it is based on the exercise of his free will.
The promisor must have chosen to be bound, and as a rational being he can choose only what he is able to carry out. This train of thought goes back to medieval moral theology and, even beyond that, to the theory of attribution of human acts contained in Aristotle's Nicomachean Ethics.'10 St. Thomas Aquinas described this connection between electio and possibilitas particularly clearly ("Et ideo voluntas completa non est nisi de possibili, quod est bonum volenti. Sed voluntas incompleta est de irnpossibili: quae secundum quosdam velleitas dicitur, quia scilicet aliquis vellet illud, si esset possibile. Electio autem nominal actum voluntatis iam determinatum ad id quod est huic agendum. Ed ideo nullo modo est nisi possibilium")[3489] and applied it to vow, oath and marriage promise. A vow (votum) is a promise made to God and it is obviously without any religious (and thus legal) significance if it involves an act that cannot possibly be carried out. An oath, too, is without much value (and thus invalid) " [s]i... est tabs res quae in eius [i.e. the person taking the oath] potestate non fuit".[3490] The impossibility is here conceived of as a vice of the will. The promise must have been made "voluntarie",[3491] which entails, inter alia, that what has been promised must be subject to the free will of the promisor. What is (to him) impossible cannot, however, be subject to a person's will. Under this perspective, incidentally, "objective" and "subjective" impossibility appear to be on a par: attention is focused on the individual promisor and his ability to carry out what he has promised. A's vow cannot become acceptable due to the fact that  or C would be able to honour it. Impossibilitas and impotentia are therefore used synonymously and entail the same legal consequences. This is apparent particularly in the canon law of marriage, where impossibilitas coeundi (impotentia in the narrow, sexual sense), a classical example of merely subjective (initial) impossibility, was regarded as an impediment to a valid marriage. The phrase coined by St. Thomas Aquinas in this context ("nullus potest se obligare ad impossibile")[3492] is more than vaguely reminiscent of Celsus' famous rule, but has, at the same time, acquired a new dimension: nobody can bind himself to do what is beyond his powers. Grotius generalized these ideas and Pufendorf further refined them.[3493] Thus, already in Grotius' Inleiding (!) we read that, since contractual obligations"vereisschen voor eerst vrij oeffening des willes,... zoo en êàï îîñê ïàå × aengebooren recht niemand hem zelve verbinden tot zaken die alle menschen, ofte hem in't byzonder, zijn onmoghelijck ofte ongeoorloft":[3494]
by natural law man cannot bind himself to things which are impossible or not permitted for men generally or for him in particular. Reason: the free will of the promisor can be directed only towards an act or a performance which is within his (personal) potestas.
7.
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