Rousseau continued his interest in music. He wrote both the words and music of his opera Le devin du village ( The Village Soothsayer ), which was performed for King Louis XV in 1752. The king was so pleased by the work that he offered Rousseau a lifelong pension. To the exasperation of his friends, Rousseau turned down the great honor, bringing him notoriety as "the man who had refused a king's pension". He also turned down several other advantageous offers, sometimes with a brusqueness bordering on truculence that gave offense and caused him problems. The same year, the visit of a troupe of Italian musicians to Paris, and their performance of Giovanni Battista Pergolesi 's La serva padrona , prompted the Querelle des Bouffons , which pitted protagonists of French music against supporters of the Italian style. Rousseau as noted above, was an enthusiastic supporter of the Italians against Jean-Philippe Rameau and others, making an important contribution with his Letter on French Music .
His most important contribution to modern intellectual culture was the principle of parsimony in explanation and theory building, known as Occam's Razor. This maxim, as interpreted by Bertrand Russell, states that if one can explain a phenomenon without assuming this or that hypothetical entity, there is no ground for assuming it, in other words that one should always opt for an explanation in terms of the fewest possible causes, factors, or variables. Though he did not realize it, this principle would eventually remove the Christian God from the domain of science. He made other significant advances in logic. Ockham wrote down in words a formulae that in propositional logic, would later be called De Morgan's Laws. He investigated ternary logic, a concept that would be taken up again in the mathematical logic of the 19th century.
For, no matter how otherwise heteroclite, they share one fundamental emblem: a common and latent pessimism . All the major departures or developments of substance within this tradition are distinguished from the classical heritage of historical materialism by the darkness of their implications or conclusions. In this respect, between 1920 and 1960, Marxism slowly changed colours in the West. The confidence and optimism of the founders of historical materialism, and of their successors, progressively disappeared. Virtually every one of the significant new themes in the intellectual muster of this epoch reveals the same diminution of hope and loss of certainty. Gramsci’s theoretical legacy was the prospect of a long war of attrition against an immensely stronger structure of capitalist power, more proof against economic collapse than had been envisaged by his predecessors — a struggle with no final clarity of outcome visible. His own life indefectibly bound to the political fate of the working class of his time and nation, Gramsci’s revolutionary temper was tersely expressed in the maxim "pessimism of the intellect, optimism of the will": once again, it was he who alone consciously perceived and controlled what was to be the timbre of a new and unheralded Marxism. The pervasive melancholy of the work of the Frankfurt School lacked any comparable note of active fortitude. Adorno and Horkheimer called in question the very idea of man’s ultimate mastery of nature, as a realm of deliverance beyond capitalism. Marcuse evoked the utopian potentiality of the liberation of nature in man, only to deny it the more emphatically as an objective tendency in reality, and to conclude that the industrial working class was itself perhaps absorbed past recall within capitalism. The pessimism of Althusser and Sartre had another but no less grave horizon, the very structure of socialism itself. Althusser declared that even communism would remain opaque as a social order to the individuals living under it, deceiving them with the perpetual illusion of their liberty as subjects. Sartre rejected the very idea of a true dictatorship of the proletariat as an impossibility, and interpreted the bureaucratization of socialist revolutions as the ineluctable product of a scarcity whose end remained inconceivable in this century.