在存在比较优势的情况下，价值不可能只是劳动时间的函数，甚至只取决于供需比例，而与劳动时间无关。所以“社会必要劳动时间”这个定义根本就不成立，马克思往下全是扯淡。
李嘉图本人就发现了这一点。这里不再提供那个举了n遍的中文链接了，改英文的：
http://homepage.newschool.edu/~het/profiles/ricardo.htm
However, Ricardo realized that when the question of capital comes in, a problem arose: specifically, as different industries apply different amounts of capital per laborer, then the rate of profit will also differ across industries. Ricardo understood that if he then assumed that the rates of profit across different industries were equalized (as free competition would imply), then, mathematically, relative prices would now vary with wages -- exactly what he had criticized Smith for! Ricardo realized that the labor theory of value would only work if the degree of capital-intensity was the same across all sectors, casting doubt on the generality of his cherished theory.