The mind assume similarity between sequential phenomena or sensory inputs when they are closely matched the mind find that a set of phenomena has, by the way it precives, resmblance to other set of phenomena, all ready stored in his memory. We then can say that there were some order in phenomena, that was resamble to other order of in the phenomena.  In a philospohical manner it is hard to define similarity in phenomena. For some solution you can go to Churchland<ref>[http://books.google.co.il/books/about/Plato_s_Camera.html?id=-QmCmfTO3TEC&redir_esc=y Paul M. Churchland. (2012). Plato’s Camera: How the Physical Brain Captures a Landscape of Abstract Universals. MIT Press.]</ref>.  When using neurons we can say that when signals from the same sets of receptors or from same set of sensory objects occur simultaneously, they will initiate an [http://en.wikipedia.org/wiki/Long-term_potentiation LTP]<ref>Teyler, T. J., & DiScenna, P. (1987). Long-term potentiation. Annual review of neuroscience, 10, 131–61.</ref> somewhere down the neural-networks, and will cause induction or LTP between the set of receptors or the set of sensory objects. In an optimal repetitions temporally sequence of similar inputs, the LTP will be strengthen. See also [http://en.wikipedia.org/wiki/Identity_%28philosophy%29 the problem of identity]