This theory has been considerably tempered since the age of Holbach. Naturalism has latterly been less interested in identifying the soul with the body, and more interested in demonstrating its dependence upon specific bodily conditions, after the manner of La Mettrie. The so-called higher faculties, such as thought and will, have been related to central or _cortical_ processes of the nervous system, processes of connection and complication which within the brain itself supplement the impulses and sensations congenitally and externally stimulated. The term "epiphenomenon" has been adopted to express the distinctness but entire dependence of the mind. Man is "a conscious automaton." The real course of nature passes through his nervous system, while consciousness attends upon its functions like a shadow, present but not efficient.[252:18] [Sidenote: Knowledge, Positivism and Agnosticism.] § 117. Holbach's "Système de la Nature," published in 1770, marks the culmination of the unequivocally materialistic form of naturalism. Its epistemological difficulties, always more or less in evidence, have since that day sufficed to discredit materialism, and to foster the growth of a critical and apologetic form of naturalism known as _positivism_ or _agnosticism_. The modesty of this doctrine does not, it is true, strike very deep. For, although it disclaims knowledge of ultimate reality, it also forbids anyone else to have any. Knowledge, it affirms, can be of but one type, that which comprises the verifiable laws governing nature. All questions concerning first causes are futile, a stimulus only to excursions of fancy popularly mistaken for knowledge. The superior certainty and stability which attaches to natural science is to be permanently secured by the savant's steadfast refusal to be led away after the false gods of metaphysics.