He was a historian whose economic ideas passed through different phases. The acceptance of free trade principles in "De la richesse commerciale" was abandoned in favour of a critical posture towards free trade and industrialisation. "Nouveaux principes d'économie politique" attacked wealth accumulation both as an end in itself, and for its detrimental effect on the poor. He indicated contradictions of capitalism. He criticized the harsh conditions endured by the workers from the standpoint of a liberal republican. He was also a passionate opponent of slavery. Adolphe Blanqui said of him: "No writer has shown a sympathy more notable and more touching for the working classes." Jean-Baptiste Say referred to Sismondi as "that enlightened author, ingenious, eloquent and selfless."