This paper proposes a diachronic and synchronic analysis of the extensive use of se in Spanish reflexive and passive constructions with se being discussed in relation to the Unaccusative Hypothesis in Arc Pair Grammar.Universals in Arc Pair Grammar, namely the Unaccusative Law, the 1-Advancement Exclusiveness Law and the Final 1-Arc Law will account for the similarities and differences, first between English and Spanish predicates; and second, between ordinary reflexive constructions and reflexive passive constructions.Evidence from Spanish, primarily synchronic, is presented to analyze reflexive constructions that come from an initial unaccusative stratum. A parallel between these constructions and reflexive passives is established to demonstrate that unaccusative predicates and reflexive passive constructions are very closely related.Diachronically the paper claims that there is a logical and clear evolution of se based on corefentiality, anaphoric chains, unspecified arguments, and self-erasing ghost arcs.Finally this paper attempts to explain why in some instances the predicate will not exhibit agreement with the corresponding nominal. The Chomeur relation and the introduction of ghost arcs according to Arc Pair Grammar Laws and Theorems will provide the basis for this explanation.