The mind assume similarity between phenomena or sensory inputs when they are closely matched order of 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 [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 repitation repetitions temporaly sequance of similar inputs, the LTP will be strength.