positivism, in particular in its conception of a philosophical system as the unification of the various branches of scientific knowledge. He also followed positivism in his insistence that it was only possible to have genuine knowledge of phenomena and hence that it was idle to speculate about the nature of the ultimate reality. The tension between positivism and his residual deism ran through the entire System of Synthetic Philosophy. Spencer followed Comte in aiming for the unification of scientific truth; it was in this sense that his philosophy aimed to be 'synthetic.' Like Comte, he was committed to the universality of natural law, the idea that the laws of nature applied without exception, to the organic realm as much as to the inorganic, and to the human mind as much as to the rest of creation. The first objective of Synthetic Philosophy was thus to demonstrate that there were no exceptions to being able to discover scientific explanations, in the form of natural laws, of all the phenomena of the universe. Spencer's volumes on biology, psychology, and sociology were all intended to demonstrate the existence of natural laws in these specific disciplines. Even in his writings on ethics, he held that it was possible to discover 'laws' of morality that had the status of laws of nature while still having normative content, a conception which can be traced to George Combe's Constitution of Man. The second objective of the Synthetic Philosophy was to show that these same laws led inexorably to progress. In contrast to Comte, who stressed only the unity of the scientific method, Spencer sought the unification of scientific knowledge in the form of the reduction of all natural laws to one fundamental law, the law of evolution. In this respect, he followed the model laid down by the Edinburgh publisher Robert Chambers in his anonymous Vestiges of the Natural History of Creation (1844). Although often dismissed as a lightweight forerunner of Charles Darwin's The Origin of Species, Chambers' book was, in reality, a programme for the unification of science which aimed to show that Laplace's nebular hypothesis for the origin of the solar system and Lamarck's theory of species transformation were both instances of 'one magnificent generalisation of progressive development' (Lewes' phrase). Chambers was associated with Chapman's salon and his work served as the unacknowledged template for the Synthetic Philosophy. Evolution Spencer first articulated his evolutionary perspective