Seven years after Time and Free Will, Henri Bergson published his major work: Matter and Memory. The decision to structure the work in such a fashion has its advantages: it allows the analysis to move in a linear way from perception of the external world to memory as the essence of mind, and then to illuminate the unity of the whole series in order to respond to the problem of dualism. For Bergson, episodic memory not only has no need for consolidation, but it cannot be consolidated; when a memory is consolidated it has become a habit. What Bergson discusses as mechanical memory stands at one end of a scale, since memory, on his account, is not just one thing but a spectrum, a continuum of capacities that shade into each other. Based on and contrasted with motor recognition is attentive recognition, which requires the "regular intervention of memory images".