Abstract: The philosophical principle known as Occam’s Razor suggests that we Should not make unnecessary a simple explanation is to be preferred to a complex one. A number of different analyses of types of understanding have deen made, starting from Recorde (1551) to the modern work of such writers as skemp, Mellin- Olsen and others. Over-elaboration of models may not be of much help to the practicing teacher. A simple division of experience into natural. conflicting and alien experiences, described by Duffin and Simpson (1994, 1995) which has much in common with the work of Piaget and Davis, may be a more fruitful approach.