Most neuroscientists and philosophers assume that memories must be stored as physical traces within brains, although the mechanism by which this happens remains elusive. But several philosophers, including Henri Bergson and Ludwig Wittgenstein have doubted that memory can work in this way, and Rupert Sheldrake has proposed that memories are better understood in terms of morphic resonance, a process whereby patterns of activity in the past resonate with patterns in the present on the basis of similarity, with this resonance passing across or through space and time from the past to the present. He has discussed this hypothesis in detail in his book The Presence of the Past and it is summarised in his book Science Set Free/The Science Delusion in Chapter 7.