Smith maintains that services, however useful, are unproductive because they are not durable and so not accumulable, and moreover because they exchange labour for revenue (i.e. wealth used for personal consumption), not for capital (wealth used for investment). The authors of this group do not deny Smith’s concept of unproductive labour but attack the weakest point of his approach, namely the idea that only material labour is productive. They also claim that intellectual labour and services are useful for the production process.