 Sensa are occurrent phenomena, and therefore fleeting (222): If I hear the bell striking the hour of four, the experience comprises many passing sensa, each no sooner generated than gone. But if I am to be aware of them as a process extended in time with particular morphology, I must retain the sound of the bell, along with the silences in between. The objects that I so retain cannot be the sensa themselves. Instead, the faculty of ‘conscious attention’ generates ‘ideas’ corresponding to the sensa, and which are retained. Sensa alone—or sense data, or impressions—are never sufficient for consciousness. Collingwood cites Kant approvingly in the connection; noting that we have always to ‘fill in’ occurrent perception to generate a complete world of 3-D objects, he writes: