In 1997 Pasinetti has re-proposed an interpretation of the principle of effective demand he first suggested in a well-known article in 1974 [see Pasinetti, 1974b and 1997]. In its current interpretation, this principle is the very core of Keynesian theory; in Pasinetti’s approach it acquires different connotations without losing any of its importance. Though Keynes was certainly aware that the principle of effective demand was the reverse of Say’s law, Pasinetti in the paper of 1997 holds that neither he nor any other economist has ever clearly stated a principle that can actually he considered the exact opposite of Say’s law. Our analysis of Pasinetti’s two studies on this subject is consequently intended as a contribution to the (ongoing) debate on the importance of Keynesian theory and its assumed revolutionary momentum.