  Black (1995) recognizes that, after 1867, Jevons' “increasing faith in the validity of Spencer’s theory of evolution was combined with decreasing faith in the validity of laissez-faire as a guide for economic policy”. Unitarians struggled with the ‘contradiction’ between the need for individual self-improvement on the one hand, and active interventionism on the other hand. This ‘contradiction’ can also be depicted as a tension existing between a belief in scientific principles on the one hand — the laws of economics — and the recognition that moral and intellectual self-improvement is impossible as long as the first necessaries of life are still out of reach. Unitarians concluded that a rational mode of living had to be imposed on the poor before they could benefit from education.  Jevons tried to resolve this paradox by restricting the scientific analysis to what can be accounted for in terms of the accumulation of wealth, and by suggesting that a ‘higher’ calculus of pleasures and pains is needed when ‘higher’ motives interfere. Interventionism could then be justified by pointing out those ‘higher’ motives. The scientific basis of Jevons' economics is utilitarianism and the mechanics of utility and self-interest; like many Unitarians this scientific belief is combined with an emphasis on active interventionism directed towards more possibilities for self-improvement; and evolutionary theory does not only show that there should be development towards the ‘good’ and the ‘happy’, but also that there will be such a development (at least in Jevons' interpretation). 