First, for Engels, the brain is a product of labour. It is in the dialectical relationship of mutual action and reaction with labour – made possible by the articulation of the hand freed by man's upright posture, the result of "hundreds of thousands of years" of natural selection – that the brain evolved to perform the most complex functions and develop self-awareness. In turn, labour is an expression of the social relations at the basis of specific socio-economic formations across successive modes of production. Therefore, the brain is a social product, mediated by the hand that wields the tool, by labour that puts it in relation with the brains of other humans, and by the language made necessary by those relations.

Second, this relationship of upright posture-hand-language-brain, and the role played by labour in humanisation, is neglected by current conceptions and is also unrecognised by Darwinian evolutionists themselves. This is because the division into classes and the separation between manual labour and intellectual labour – in the latter's connection with the ruling class and its apparatus (schools, the Churches, etc.) – has led to the prevalence of the idealist conception that gives prominence to the brain and its representations.

Third, humanity has applied science to nature, and nature has thereby been modified through science, technology, and labour. But it has been difficult to go beyond the initial and immediate effects of this transformation, barely taking into consideration its secondary and long-term effects.

Fourth, and even more importantly, science has not been applied to the social consequences of labour and to its implications, from the division of labour to class struggle, thus depriving humanity of a vision of the wider, long-term effects of productive activity. Bourgeois economic science, i.e., classical political economy, has been unable to go any further. In order to regulate human labour activity according to a truly conscious plan, knowledge is not enough; an overturning of the mode of production and social order is necessary.

Fifth, the individual capitalist is concerned only with immediate profit, not with what will happen to the commodity or the buyer afterwards. The same applies to what will happen to nature and – we might add – to human nature itself.